U.S. patent application number 12/980276 was filed with the patent office on 2011-12-01 for low cost solar cells formed using a chalcogenization rate modifier.
Invention is credited to Katherine Dickey, David B. Jackrel, Jacob Woodruff.
Application Number | 20110294254 12/980276 |
Document ID | / |
Family ID | 44307488 |
Filed Date | 2011-12-01 |
United States Patent
Application |
20110294254 |
Kind Code |
A1 |
Jackrel; David B. ; et
al. |
December 1, 2011 |
LOW COST SOLAR CELLS FORMED USING A CHALCOGENIZATION RATE
MODIFIER
Abstract
Methods and devices are provided for forming an absorber layer.
In one embodiment, a method is provided comprising of depositing a
precursor material onto a substrate, wherein the precursor material
may include or may be used with an additive to minimize
concentration of group IIIA material such as Ga in the back portion
of the final semiconductor layer. The additive may be a non-copper
Group IB additive in elemental or alloy form.
Inventors: |
Jackrel; David B.;
(Pacifica, CA) ; Dickey; Katherine; (Stanford,
CA) ; Woodruff; Jacob; (Mountain View, CA) |
Family ID: |
44307488 |
Appl. No.: |
12/980276 |
Filed: |
December 28, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61290490 |
Dec 28, 2009 |
|
|
|
Current U.S.
Class: |
438/95 ;
257/E31.004; 438/478 |
Current CPC
Class: |
H01L 21/02601 20130101;
H01L 21/02614 20130101; Y02E 10/541 20130101; Y02P 70/50 20151101;
H01L 21/02628 20130101; H01L 31/0322 20130101; Y02P 70/521
20151101; H01L 21/02568 20130101 |
Class at
Publication: |
438/95 ;
257/E31.004; 438/478 |
International
Class: |
H01L 31/0264 20060101
H01L031/0264 |
Claims
1. A method comprising: formulating an ink of particles wherein
about 50% or more of the particles are flakes each containing at
least one element from group IB, IIIA and/or VIA and having a
non-spherical, planar shape, wherein overall amounts of elements
from group IB, IIIA and/or VIA contained in the ink are such that
the ink has a desired stoichiometric ratio of the elements, wherein
at least some of the particles includes a material that is a
chalcogenization rate modifier selective to chalcogenation of one
or more group IIIA elements; coating a substrate with the ink in
one or more steps to form a precursor layer; heating the precursor
layer form a densified layer wherein the chalcogenization rate
modifier binds with a group IIIA material; processing the densified
layer in one or more steps in a process gas atmosphere to form a
desired semiconductor absorber layer.
2. The method of claim 1 wherein chalcogenization rate modifier is
selective for indium.
3. The method of claim 1 wherein chalcogenization rate modifier is
selective for gallium.
4. The method of claim 1 wherein the process gas atmosphere
involves using a selenium atmosphere.
5. The method of claim 1 wherein processing involves using a
selenium-based atmosphere and then a sulfur-based atmosphere.
6. The method of claim 1 wherein processing involves using a
selenium-based atmosphere and a sulfur-based atmosphere.
7. The method of claim 1 wherein the chalcogenization rate modifier
forms a silver-group IIIA alloy phase in the densified layer.
8. The method of claim 1 wherein the chalcogenization rate modifier
forms a gold-group IIIA alloy phase in the densified layer.
9. The method of claim 1 wherein limiting availability of free,
elemental indium in the densified layer by binding the
chalcogenization rate modifier to form a silver-IIIA alloy
phase.
10. The method of claim 6 wherein the chalcogenization rate
modifier alloy phase is formed in localized areas in a repeating or
other pattern in the densified layer.
11. The method of claim 6 wherein the chalcogenization rate
modifier alloy phase is concentrated in islands of material in the
densified layer.
12. The method of claim 6 wherein the densified layer includes
chalcogenization rate modifier-indium phase and areas of IB-gallium
alloy phase in distributed patterns over the substrate.
13. The method of claim 6 wherein heating the precursor creates
segregated chalcogenization rate modifier-indium phase and areas of
IB-gallium alloy phase over the substrate.
14. The method of claim 6 wherein the ink is formed without group
VIA material therein.
15. A method comprising: forming a precursor layer having a
chalcogenization rate modifier and group IB, IIIA and/or VIA
elements on a substrate; heating the precursor layer to form a
densified layer with segregated areas of the chalcogenization rate
modifier-group IIIA alloy phase and areas of copper-gallium alloy
phase; processing the densified layer in one or more steps in a
process gas atmosphere to form a desired semiconductor absorber
layer.
16. The method of claim 13 wherein upper surfaces of the areas of
copper-gallium alloy phase are exposed to increase surface area
exposure of copper-gallium alloy phase during group VIA
processing.
17. The method of claim 1 wherein limiting availability of free,
elemental indium in the densified layer by binding the
chalcogenization rate modifier to form a non-copper, Group
IB-indium alloy phase.
18. The method of claim 13 wherein the non-copper, Group IB-indium
alloy phase comprises Au--In.
19. The method of claim 13 wherein non-copper, Group IB-indium
alloy phase comprises Ag--In.
20. A method comprising: forming a precursor layer having a
chalcogenization rate modifier and group IB, IIIA and/or VIA
elements on a substrate; heating the precursor layer to form a
densified layer with segregated areas of the chalcogenization rate
modifier-group IIIA alloy phase and areas of copper-gallium alloy
phase, wherein indium is present only in elemental form or as part
of the chalcogenization rate modifier-group IIIA alloy phase;
processing the densified layer in one or more steps in a process
gas atmosphere to form a desired semiconductor absorber layer,
wherein the precursor layer exposed surface area for copper-gallium
alloy phase is much greater relative to a densified layer without
chalcogenization rate modifier but is otherwise identical.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application Ser. No. 61/290,490 filed Dec. 28, 2009 and fully
incorporated herein by reference for all purposes.
FIELD OF THE INVENTION
[0002] This invention relates generally to photovoltaic devices,
and more specifically, to use of additives or dopants during
photovoltaic device manufacturing for enhanced bandgap grading.
BACKGROUND OF THE INVENTION
[0003] Solar cells and solar modules convert sunlight into
electricity. These electronic devices have been traditionally
fabricated using silicon (Si) as a light-absorbing, semiconducting
material in a relatively expensive production process. To make
solar cells more economically viable, solar cell device
architectures have been developed that can inexpensively make use
of thin-film, preferably non-silicon, light-absorbing semiconductor
materials such as but not limited to copper-indium-gallium-selenide
(CIGS).
[0004] Many traditional thin-film CIGS manufacturing techniques use
co-evaporation or other vacuum based deposition techniques where
all of the components of the final semiconductor material are
formed in one step. In particular for co-evaporation, the material
is grown from the bottom-up, with content carefully controlled as
the material is grown. Although material content through the depth
of the layer is more controllable, this one step type fabrication
process is typically a time consuming process.
[0005] By contrast, multi-step fabrication techniques which involve
deposition and then a subsequent anneal in one or more steps in a
group VIA or other reactive environment can sometimes be a higher
throughput process that, unfortunately, is more susceptible to
migration and/or phase separation of material during fabrication.
In one nonlimiting example, gallium content in depth of the
initially deposited material is subsequently much different through
the depth of the final semiconductor layer as much of gallium is
pushed to bottom of the layer in the final semiconductor
material.
[0006] It should be understood that the reaction thermodynamics and
kinetics of 2-step selenization and/or sulfidation approaches for
Cu(In,Ga)(Se,S)2 from metallic precursors favor the reaction of Cu
and In with selenium, before Ga reacts with selenium. This causes
Ga to be pushed to the back of the film. There are some advantages
to having some Ga at the back, such as the formation of a back
surface field. However, having some Ga at the front or top surface
is beneficial to solar cell performance because Ga raises the
bandgap of CIG(S)Se, and the bandgap at the front of the film
greatly influences the open-circuit voltage of the solar cell.
[0007] A number of groups have published work on ACIGS. Recently,
the group of Bill Shafarman (IEC, U. Delaware) have published work
regarding the co-evaporation of ACIGS, where they have achieved
results in lowering the processing temperature for their
coevaporation process. However, very little if any of the published
work relates to two-step (i.e. selenization) of ACIG-based
precursors.
[0008] A central challenge in cost-effectively constructing a
large-area CIGS-based solar cell or module involves reducing
processing costs and material costs. Most of the more
cost-effective deposition systems use a non-vacuum deposition
technique such as printing an ink of a precursor material or other
non-vacuum coating techniques. Most of these processes which do not
lock in the material profile such that of co-evaporation, are faced
with material migration and/or phase separation issue associated
with using more than one processing step. Much of the challenge for
CIGS or other group IB-IIIA-VIA absorbers, involves how to keep the
group IIIA material from concentrating in the back or bottom of the
layer.
[0009] One problem faced with the selenization type processes or
two-stage processes to grow CIGS is the difficulty of distributing
Ga uniformly through the thickness of the absorber layer formed
after reaction of Cu, In and Ga containing metallic precursor film
with Se. It is believed that when a metallic precursor film
including Cu, In and Ga is deposited first on a contact layer (such
as Mo) of a base and then reacted with Se, the Ga-rich phases
segregate to the film/base interface (or the film/contact layer
interface) because reactions between Ga-bearing species and Se are
slower than the reactions between In-bearing species and Se.
[0010] Due to the aforementioned issues, improved techniques are
desired so that improved photovoltaic absorbers are formed.
Improvements may be made to increase quality of of CIGS/CIGSS
manufacturing processes and improve the performance associated with
CIGS/CIGSS based solar devices.
SUMMARY OF THE INVENTION
[0011] Embodiments of the present invention address at least some
of the drawbacks set forth above. It should be understood that at
least some embodiments of the present invention may be applicable
to any type of solar cell, whether they are rigid or flexible in
nature or the type of material used in the absorber layer.
Embodiments of the present invention may be adaptable for
roll-to-roll and/or batch manufacturing processes for forming Group
IBIIIAVIA compound semiconductors that contain Group IB (Cu, Ag,
Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te,
Po). At least some of these and other objectives described herein
will be met by various embodiments of the present invention.
[0012] For any of the embodiments herein, it is also possible to
have two or more elements of IB elements in the chalcogenide
particle and/or the resulting intermediate film and/or final
absorber layer. One notable problem encountered when creating
CIG(S)Se films for solar cells using low cost 2-step selenization
methods is preventing Ga from segregating to the back of the film.
Two-step selenization methods entail one step where a metal, oxide,
or chalcogenide (or other Ag, Au, Cu, In, Ga, S, or Se containing
compounds) precursor film is deposited, and then subsequently
selenized (and/or sulfidized) in a second step. The approach
described herein is to create (Ag,Cu)(In,Ga)(Se,S).sub.2 (hereafter
referred to as ACIGS) by adding Ag to the precursor film. It has
been found that this enables more of the Ga to remain forward in
the film. Ga at the front surface of the film allows for higher
open-circuit voltage solar cells and therefore higher
efficiencies.
[0013] Optionally, the following may also be adapted for use with
any of the embodiments disclosed herein. Processing comprises
annealing with a ramp-rate of 1-5.degree. C./sec, preferably over
5.degree. C./sec, to a temperature of about 225 to 575.degree. C.
Optionally, processing comprises annealing with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225 to 575.degree. C. preferably for about 30
seconds to about 600 seconds to enhance conversion of indium
hydroxide or other hydroxide, densification and/or alloying between
Cu, In, and Ga in an atmosphere containing hydrogen gas, where the
plateau temperature not necessarily is kept constant in time.
Optionally, processing further comprises selenizing and/or
sulfidizing this annealed layer with a ramp-rate of 1-5.degree.
C./sec, preferably over 5.degree. C./sec, to a temperature of about
225 to 575.degree. C. for a time period of about 60 seconds to
about 10 minutes in the appropriate VIA vapor, where the plateau
temperature not necessarily is kept constant in time, to form the
thin-film containing one or more chalcogenide compounds containing
Cu, In, Ga, and Se. Optionally, processing comprises selenizing
and/or sulfiziding without the separate annealing step in an
atmosphere containing hydrogen gas, but may be densified and
selenized in one step with a ramp-rate of 1-5.degree. C./sec,
preferably over 5.degree. C./sec, to a temperature of 225 to
575.degree. C. for a time period of about 120 seconds to about 20
minutes in an atmosphere containing either H.sub.2Se or a mixture
of H.sub.2 and Se vapor (or H.sub.2S or H.sub.2 and S vapor).
[0014] For any of the embodiments herein, these layers can be
deposited by techniques such as but not limited to electroplating,
nanoparticle deposition, coevaporation, deposition by gas, vacuum,
or vapor phase techniques (vacuum evaporation, sputtering, vapor
transport, etc.) of the constituent elements either in sequence or
the simultaneous deposition of two or more elements, or any
combination thereof, or the like.
[0015] An additional benefit of controlling the chalcogenization
reaction rate is that it allows for the optimization of the rate of
crystallization to achieve good crystal quality with as fast a rate
as possible (to reduce the associated manufacturing costs). It is
common knowledge that crystals which are grown too fast can have
poor quality, specifically in the form of small grains and many
point defects, due to insufficient time for grain growth and the
atoms forming the growing crystal not having enough time to find
the appropriate positions in the lattice. On the other hand, if
crystallization occurs very slowly, then the material becomes
costly to manufacture. A crystallization rate modifier is useful to
slow down reactions that occur too quickly to create high quality
crystals, or speed up reactions that occur too slowly to be cost
effective in production. In embodiments of the present invention,
the chalcogenization rate modifier is compound selective, such that
it promotes the preferential chalcogenization of one species over
another in the crystal growth.
[0016] In one embodiment of the present invention, a method is
provided comprising formulating an ink of particles wherein about
50% or more of the particles are flakes each containing at least
one element from group IB, IIIA and/or VIA and having a
non-spherical, planar shape, wherein overall amounts of elements
from group IB, IIIA and/or VIA contained in the ink are such that
the ink has a desired stoichiometric ratio of the elements, wherein
at least some of the particles includes a material that is a
chalcogenization rate modifier selective to chalcogenation of one
or more group IIIA elements; coating a substrate with the ink in
one or more steps to form a precursor layer; and heating the
precursor layer form a densified layer wherein the chalcogenization
rate modifier binds with a group IIIA material. The densified layer
may be processed in one or more steps in one or more process gas
atmospheres to form a desired semiconductor absorber layer
[0017] For any of the embodiments herein, the embodiments can be
configured to incorporate one or more of the following. By way of
example, the processing of the densified film involves using a
group VIA atmosphere. Optionally, the group VIA reactive processing
involves using a selenium atmosphere. Optionally, the group VIA
reactive processing involves using a selenium-based atmosphere and
a sulfur-based atmosphere. Optionally, the chalcogenization rate
modifier comprises a silver-group IIIA alloy phase in the densified
layer that comprises at least 90% of the silver in the layer.
Optionally, the method involves limiting availability of free,
elemental indium in the densified layer by binding the
chalcogenization rate modifier to form a silver-indium alloy phase.
Optionally, the method involves limiting availability of free,
elemental indium in the densified layer by binding the
chalcogenization rate modifier to form a silver-IIIA alloy phase.
Optionally, the method involves limiting availability of free,
elemental indium in the densified layer by binding the
chalcogenization rate modifier to form a silver-indium alloy phase.
Optionally, the silver-IIIA alloy phase is formed in localized
areas in a repeating pattern in the densified layer. Optionally,
the silver-indium alloy phase is concentrated in islands of
material in the densified layer. Optionally, the densified layer
includes areas of silver-indium alloy phase and areas of
copper-gallium alloy phase in distributed patterns over the
substrate. Optionally, heating the precursor creates segregated
areas of silver-indium alloy phase and areas of copper-gallium
alloy phase over the substrate. Optionally, the particles in the
ink include flake particles with silver-indium alloy phase.
Optionally, the ink is formed without group VIA material
therein.
[0018] In another embodiment of the present invention, a method is
provided comprising forming a precursor layer having a
chalcogenization rate modifier and group IB, IIIA and/or VIA
elements on a substrate; heating the precursor layer to form a
densified layer with segregated areas of the chalcogenization rate
modifier-group IIIA alloy phase and areas of copper-gallium alloy
phase; processing the densified layer in a suitable atmosphere to
form a desired semiconductor absorber layer.
[0019] For any of the embodiments herein, the embodiments can be
configured to incorporate one or more of the following. For
example, upper surfaces of the areas of copper-gallium alloy phase
are exposed in the densified layer to increase surface area
exposure of copper-gallium alloy phase during group VIA processing.
Optionally, some embodiments can minimize the amount of
copper-indium present in the densified layer. Optionally, the
method includes limiting availability of free, elemental indium in
the densified layer by binding the chalcogenization rate modifier
to form a non-copper, Group IB-indium alloy phase. Optionally, the
non-copper, Group IB-indium alloy phase comprises Au--In.
Optionally, the non-copper, Group IB-indium alloy phase comprises
Ag--In.
[0020] In yet another embodiment of the present invention, the
method comprises forming a precursor layer having a
chalcogenization rate modifier and group IB, IIIA and/or VIA
elements on a substrate; heating the precursor layer to form a
densified layer with segregated areas of the chalcogenization rate
modifier-group IIIA alloy phase and areas of copper-gallium alloy
phase, wherein indium is present only in elemental form or as part
of the chalcogenization rate modifier-group IIIA alloy phase;
processing the densified layer in a suitable atmosphere to form a
desired semiconductor absorber layer. Optionally, the precursor
layer exposed surface area for copper-gallium alloy phase is much
greater relative to a densified layer without chalcogenization rate
modifier but is otherwise identical.
[0021] For any of the embodiments herein, the embodiments can be
configured to incorporate one or more of the following. For
example, the chalcogenization rate modifier is selective for
gallium. Without being tied to any particular theory, some
embodiments may speed up gallium processing. Some may increase
gallium exposure by causing greater surface area exposure of
IB-IIIA areas on the densified film. Optionally, the
chalcogenization rate modifier is selective for indium. Without
being tied to any particular theory, some embodiments may delay
indium processing by decreasing the amount of copper-IIIA or
copper-indium in the densified film. Some embodiments completely
remove all copper indium in the densified layer. Some embodiments
may leave 10 wt % or less of what would have been there without the
rate modifier. Optionally, the process gas atmosphere involves
using a selenium atmosphere. Optionally, processing involves using
a selenium-based atmosphere and then a sulfur-based atmosphere.
Optionally, processing involves using a selenium-based atmosphere
and a sulfur-based atmosphere. Optionally, chalcogenization rate
modifier forms a silver-group IIIA alloy phase in the densified
layer. Optionally, chalcogenization rate modifier forms a
gold-group IIIA alloy phase in the densified layer. Optionally, the
method involves limiting availability of free, elemental indium in
the densified layer by binding the chalcogenization rate modifier
to form a silver-IIIA alloy phase. Optionally, the chalcogenization
rate modifier alloy phase is formed in localized areas in a
repeating or other pattern in the densified layer. Optionally, the
chalcogenization rate modifier alloy phase is concentrated in
islands of material in the densified layer. Optionally, the
densified layer includes chalcogenization rate modifier-indium
phase and areas of IB-gallium alloy phase in distributed patterns
over the substrate. Optionally, heating the precursor creates
segregated chalcogenization rate modifier-indium phase and areas of
IB-gallium alloy phase over the substrate.
[0022] A further understanding of the nature and advantages of the
invention will become apparent by reference to the remaining
portions of the specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIGS. 1A-1D are schematic cross-sectional diagrams
illustrating fabrication of a film according to an embodiment of
the present invention.
[0024] FIGS. 2A-2F show a series of cross-sectional views showing
formation of various layers of materials according to one
embodiment of the present invention.
[0025] FIG. 3 shows a flowchart of a method according to one
embodiment of the present invention.
[0026] FIGS. 4A-4C show various views of non-spherical particles
according to embodiments of the present invention.
[0027] FIGS. 5A-5C show the formation of a semiconductor layer from
a precursor layer comprised of spherical and non-spherical
particles.
[0028] FIG. 6 shows a top down view of annealed material according
to one embodiment of the present invention.
[0029] FIG. 7 shows a schematic of a roll-to-roll manufacturing
system according to the one embodiment of the present
invention.
[0030] FIG. 7A shows a schematic of a system using a chalcogen
vapor environment according to one embodiment of the present
invention.
[0031] FIG. 7B shows a schematic of a system using a chalcogen
vapor environment according to one embodiment of the present
invention.
[0032] FIG. 7C shows a schematic of a system using a chalcogen
vapor environment according to one embodiment of the present
invention.
[0033] FIG. 8 shows a side cross-sectional view of a solar cell
according to one embodiment of the present invention.
[0034] FIGS. 9A-9B shows side cross-sectional views of material for
use in forming solar cells according to embodiments of the present
invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0035] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention, as
claimed. It may be noted that, as used in the specification and the
appended claims, the singular forms "a", "an" and "the" include
plural referents unless the context clearly dictates otherwise.
Thus, for example, reference to "a material" may include mixtures
of materials, reference to "a compound" may include multiple
compounds, and the like. References cited herein are hereby
incorporated by reference in their entirety, except to the extent
that they conflict with teachings explicitly set forth in this
specification.
[0036] In this specification and in the claims which follow,
reference will be made to a number of terms which shall be defined
to have the following meanings:
[0037] "Optional" or "optionally" means that the subsequently
described circumstance may or may not occur, so that the
description includes instances where the circumstance occurs and
instances where it does not. For example, if a device optionally
contains a feature for an anti-reflective film, this means that the
anti-reflective film feature may or may not be present, and, thus,
the description includes both structures wherein a device possesses
the anti-reflective film feature and structures wherein the
anti-reflective film feature is not present.
[0038] In one embodiment of the present invention, an additive
and/or dopant is added to the precursor material to minimize the
migration of the group IIIA material to the back of the absorber
layer. As mentioned, when selenizing and/or sulfadizing metallic
(or oxide, or chalcogenide, or other Cu, In, Ga, S, or Se
containing compounds) precursor films, it is difficult to keep the
Ga from segregating to the back of the film. The additive or dopant
is added to minimize Ga segregation to the back of the layer.
[0039] In one non-limiting example, it has been found that the
addition of an additional group IB material other than copper can
result in less Ga segregation in depth. Without being limited to
any one theory, two mechanisms are that 1) the group IB additive
slows down the reaction responsible for the Ga segregation since
the IB additive is a more noble metal, or 2) the IB additive speeds
up the reaction of a Ga-containing intermediate product which then
keeps the Ga from segregating to the back of the finished ACIGS
film.
[0040] With this in mind, one embodiment of the present invention
uses a Group IB additive of silver (Ag) so that low cost, high
throughput processing methods are used to create ACIGS, including
nanoparticle printing of precursor materials on an inexpensive
flexible metal substrate followed by a rapid thermal process to
form the large grain high quality ACIGS material. The product was
ACIGS material with higher minimum bandgaps than produced using
similar processing for CIGS (without Ag).
[0041] In some embodiments, the Ag or group IB additive is evenly
distributed in the ink during deposition. This even distribution in
the ink makes the silver or group IB additive available throughout
the precursor layer for reaction to all the particles and/or
precursors therein. This provides the group IB additive material
throughout the depth of the layer and not concentrated at any
particular depth. Some embodiments may use particles of smaller
sizes such as from 0.1 nm to 1500 nm in mean diameter or largest
dimension. Optionally, the particle sizes is between 0.5 nm to 200
nm in mean diameter or largest dimension. Optionally, the particle
sizes is between 0.5 nm to 100 nm in mean diameter or largest
dimension. Optionally, the particle sizes is between 0.5 nm to 50
nm in mean diameter or largest dimension. Optionally, the particle
sizes is between 0.5 nm to 20 nm in mean diameter or largest
dimension.
[0042] Optionally, other particle shapes such as spherical, flake,
hexagonal, polygonal, cylindrical, nanowires, or combinations of
any of the foregoing may also be used. For example, methods have
been used to provide uniform layers of nanowires over semiconductor
materials. Optionally, some embodiment may desire to lock in the IB
material be using core-shell particles with IIB in the core and a
IB, IIIA, or VIA shell.
[0043] In some embodiments, it should be understood that all of
particles used in the system are solid particles. Optionally, only
one of the components is in liquid form above 100.degree. C.
[0044] Optionally, at least two of the particles are binary alloys.
By way of nonlimiting example, the material may be
Cu(solid)+In(solid)+Ga(liquid above 100.degree. C.)+Ag(solid) in
the precursor. Optionally, the material may be
CuIn(solid)+Ga(liquid above 100.degree. C.)+Ag(solid) in the
precursor. Optionally, the material may be
Cu(solid)+CuGa(solid)+In(solid)+Ag(solid) in the precursor.
Optionally, the material may be CuIn(solid)+CuGa(solid)+Ga(liquid
above 100.degree. C.)+Ag(solid) in the precursor. Some embodiments
may substitute Ag-IIIA, Ag--Cu-IIIA, Ag--Cu--In--Ga, AgIn.sub.2,
AgGa, and/or (Cu Ga).sub.1In.sub.2 for the In or Ga source.
[0045] These various precursor combinations can be used as stated
or in combination with other elements or materials to achieve a
material with the following stoichiometric combinations:
Ag/(Ag+Cu)=0.05-0.6, Ga/(Ga+In)=0.1-0.7, and
(Ag+Cu)/(Ga+In)=0.7-1.0. Optionally, the various precursor
combinations can be used as stated or in combination with other
elements or materials to achieve a material with the following
stoichiometric combinations: Ag/(Ag+Cu)=0.1-0.4,
Ga/(Ga+In)=0.2-0.5, and (Ag+Cu)/(Ga+In)=0.8-0.9. The resulting
devices with the group IB additive had high Voc=700 mV. Optionally,
the resulting devices with the group IB additive had high Voc of at
least 650 mV. Optionally, the resulting devices with the group IB
additive had high Voc of at least 600 mV. Optionally, the resulting
devices with the group IB additive had high Voc of at least 550
mV.
[0046] In one embodiment with Ag/(Ag+Cu)=0.05-0.6,
Ga/(Ga+In)=0.1-0.7, and (Ag+Cu)/(Ga+In)=0.7-1.0, the final absorber
has about 25% gallium up front with a bandgap of about 1.2 eV. This
is a desirable bandgap for CIGS. Optionally, some embodiments may
use a higher silver content and a lower gallium content than those
recited above. This may achieve the same bandgap at the front, but
there will be less of a gradient in the bandgap. The gallium curve
will be flatter due to the lower gallium content. The silver or
group IB additive makes the gallium flatter in depth. In one
embodiment, there may be 20% gallium up front and 70% in the back.
Other embodiments may have about 40% in the back. 40% flat for
gallium is too high. Thus, a flatter gallium profile at a lower
level such that a 25% gallium content in the front with a flatter
profile is desirable.
[0047] Optionally, some embodiments may use a ratio of about
Ag/(Ag+Cu)=0.25-0.30, Ga/(Ga+In)=0.25-0.33, and
(Ag+Cu)/(Ga+In)=0.80-0.95. The bandgap is not linear through the
depth of the layer. Silver is everywhere throughout the layer. The
silver will pull up the bandgap. In some embodiments, the silver
content is substantially constant through the depth of the absorber
layer. In some embodiments, the bandgap probably may not be
perfectly flat, and thus it may be desirable to reduce the overall
gallium content so that the gallium at the rear or back of the
layer is not too high.
[0048] Optionally, some embodiments may use a ratio of about
Ag/(IB)=0.05-0.4, Ga/(Ga+In)=0.30-0.80, and
(Ag+Cu)/(Ga+In)=0.80-0.89.
[0049] Optionally, some embodiments may use a ratio of about
Ag/(IB)=0.1-0.2, Ga/(Ga+In)=0.35-0.80, and
(Ag+Cu)/(Ga+In)=0.80-0.89.
Embodiment 1
[0050] A metal foil substrate was sputtered with Mo to form a 500
nm to 1000 nm Mo film to serve as the back contact. A barrier layer
of 50 nm to 300 nm of Chromium, TiN, HfN, or other transition metal
nitride barrier was formed onto the Mo back contact. A precursor
ink comprised of Cu, In, Ga, and Ag, and atomic ratios
Ag/(Ag+Cu)=0.1-0.3, Ga/(Ga+In)=0.35-0.5, and
(Ag+Cu)/(Ga+In)=0.80-1.0. For the present embodiment, an
approximately 0.5-2.5 micron thick layer of the precursor material
containing solution is deposited on the substrate. The precursor
material may be dispersed in a solvent such as water, alcohol or
ethylene glycol with the aid of organic surfactants and/or
dispersing agents described herein to form an ink.
[0051] The precursor layer is annealed with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225.degree. to about 575.degree. C. preferably
for about 30 seconds to about 600 seconds to enhance densification
and/or alloying between Cu, In, and Ga in an atmosphere containing
hydrogen or nitrogen gas, where the plateau temperature not
necessarily is kept constant in time. Some embodiments may heat to
a temperature of at least 500.degree. C. Optionally, some
embodiments may heat to a temperature of at least 505.degree. C.
Optionally, some embodiments may heat to a temperature of at least
510.degree. C. Optionally, some embodiments may heat to a
temperature of at least 515.degree. C. Optionally, some embodiments
may heat to a temperature of at least 520.degree. C. Optionally,
some embodiments may heat to a temperature of at least 525.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 530.degree. C. Optionally, some embodiments may heat to a
temperature of at least 535.degree. C. Optionally, some embodiments
may heat to a temperature of at least 540.degree. C. Optionally,
some embodiments may heat to a temperature of at least 545.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 550.degree. C.
[0052] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Some
embodiments may heat to a temperature of at least 500.degree. C.
Optionally, some embodiments may heat to a temperature of at least
505.degree. C. Optionally, some embodiments may heat to a
temperature of at least 510.degree. C. Optionally, some embodiments
may heat to a temperature of at least 515.degree. C. Optionally,
some embodiments may heat to a temperature of at least 520.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 525.degree. C. Optionally, some embodiments may heat to a
temperature of at least 530.degree. C. Optionally, some embodiments
may heat to a temperature of at least 535.degree. C. Optionally,
some embodiments may heat to a temperature of at least 540.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 545.degree. C. Optionally, some embodiments may heat to a
temperature of at least 550.degree. C.
[0053] Optionally, instead of this two-step approach, the layer of
precursor material may be selenized without the separate annealing
step in an atmosphere containing hydrogen or nitrogen gas, but may
be densified and selenized in one step with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of 225 to 600.degree. C. for a time period of about 120
seconds to about 20 minutes in an atmosphere containing either H2Se
or a mixture of H2 and Se vapor. Some embodiment use only Se
material and completely avoid H2Se. It should be understood that
other embodiments may be configured to include S vapor or H2S to
create the desired CIGS or CIGSS absorber.
[0054] These absorber layer films were then used in the fabrication
of photovoltaic devices by chemical bath deposition of 50 nm CdS,
followed by sputter deposition of an i:ZnO layer and Al:ZnO window
layer, followed by deposition of a silver paste grid structure. For
the window layer, some embodiments may use ITO, B:ZnO, A:ZnoY,
non-vacuum deposited transparent conductors or other transparent
oxides.
Embodiment 2
[0055] A metal foil substrate was sputtered with Mo to form a 500
nm to 1000 nm Mo film to serve as the back contact. A barrier layer
of 50 nm to 300 nm of Chromium, TiN, HfN, or other transition metal
nitride barrier was formed onto the Mo back contact. A precursor
ink comprised of Cu, In, Ga, and Ag, and atomic ratios
Ag/(Ag+Cu)=0.2-0.3, Ga/(Ga+In)=0.3-0.4, and
(Ag+Cu)/(Ga+In)=0.8-0.9. For the present embodiment, an
approximately 0.5-2.5 micron thick layer of the precursor material
containing solution is deposited on the substrate. The precursor
material may be dispersed in a solvent such as water, alcohol or
ethylene glycol with the aid of organic surfactants and/or
dispersing agents described herein to form an ink.
[0056] The precursor layer is annealed with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225.degree. to about 575.degree. C. preferably
for about 30 seconds to about 600 seconds to enhance densification
and/or alloying between Cu, In, and Ga in an atmosphere containing
hydrogen or nitrogen gas, where the plateau temperature not
necessarily is kept constant in time. Some embodiments may heat to
a temperature of at least 500.degree. C. Optionally, some
embodiments may heat to a temperature of at least 505.degree. C.
Optionally, some embodiments may heat to a temperature of at least
510.degree. C. Optionally, some embodiments may heat to a
temperature of at least 515.degree. C. Optionally, some embodiments
may heat to a temperature of at least 520.degree. C. Optionally,
some embodiments may heat to a temperature of at least 525.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 530.degree. C. Optionally, some embodiments may heat to a
temperature of at least 535.degree. C. Optionally, some embodiments
may heat to a temperature of at least 540.degree. C. Optionally,
some embodiments may heat to a temperature of at least 545.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 550.degree. C.
[0057] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Some
embodiments may heat to a temperature of at least 500.degree. C.
Optionally, some embodiments may heat to a temperature of at least
505.degree. C. Optionally, some embodiments may heat to a
temperature of at least 510.degree. C. Optionally, some embodiments
may heat to a temperature of at least 515.degree. C. Optionally,
some embodiments may heat to a temperature of at least 520.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 525.degree. C. Optionally, some embodiments may heat to a
temperature of at least 530.degree. C. Optionally, some embodiments
may heat to a temperature of at least 535.degree. C. Optionally,
some embodiments may heat to a temperature of at least 540.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 545.degree. C. Optionally, some embodiments may heat to a
temperature of at least 550.degree. C.
[0058] Optionally, instead of this two-step approach, the layer of
precursor material may be selenized without the separate annealing
step in an atmosphere containing hydrogen or nitrogen gas, but may
be densified and selenized in one step with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of 225 to 600.degree. C. for a time period of about 120
seconds to about 20 minutes in an atmosphere containing either H2Se
or a mixture of H2 and Se vapor. It should be understood that other
embodiments may be configured to include S vapor or H2S to create
the desired CIGS or CIGSS absorber.
[0059] These absorber layer films were then used in the fabrication
of photovoltaic devices by chemical bath deposition of 50 nm CdS.
The parts were finished into devices using a CdS junction partner,
a ZnO-based transparent conductor and a metallic grid top contact
formed from a silver based paste. For the window layer, some
embodiments may use ITO, B:ZnO, A:ZnoY, non-vacuum deposited
transparent conductors or other transparent oxides.
Embodiment 3
[0060] A metal foil substrate was sputtered with Mo to form a 500
nm to 1000 nm Mo film to serve as the back contact. A barrier layer
of 50 nm to 300 nm of Chromium, TiN, HfN, or other transition metal
nitride barrier was formed onto the Mo back contact. The method may
be used to form a IB-IIB-IVA-VIA absorber material. A precursor ink
is provided wherein Ag/IB=0.1-0.4. A method of forming
(Ag,Cu).sub.xZn.sub.ySn.sub.z, (Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a
(ACZTS), (Ag,Cu).sub.xZn.sub.ySn.sub.zSe.sub.b (ACZTSe) or
(Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.aSe.sub.b (ACZTSSe) layers with
well-defined total bulk stoichiometries, wherein x ranges from 1.5
to 2.5, y ranges from 0.9 to 1.5, z ranges from 0.5 to 1.1, a
ranges from 0 to 4.2, preferably from 0.1 to 4.2, and b ranges from
0 to 4.2, preferably from 0.1 to 4.2, and which method is easy to
apply and suitable for large scale production of thin film solar
cells.
[0061] In one embodiment, the stoichiometric ratio for a ACZTS
solar cell precursor foil may be (Ag: 10 at.-%, Cu: 40 at.-%, Zn:
25 at.-%, Sn: 25 at.-%) Optionally, a further object of the
invention is to provide a thin film of CZTS (Cu.sub.2ZnSnS.sub.4)
and related compounds like
(Cu,Ag).sub.xZn.sub.ySn.sub.zCh1.sub.aCh2.sub.b. A still further
object of the present invention is to provide precursors of these
chalcogenides, i.e., more specifically, Cu.sub.xZn.sub.ySn.sub.z in
foil form. In this manner, the desired stoichiometry is fixed in
the bulk material.
[0062] It should be understood that the precursor layer is designed
to be IIB rich. This is particularly true if the IIB material is
Zn. The desirable atomic ratio of IIB to IVA is at least 55:45 or
optionally as high as 60:40. This IIB rich ratio in the precursor
is particularly desirable due to the nature of the processing that
occurs. Using particles such as but not limited to elemental
particles, the ratio of IIB material to others is controllable.
Optionally, some embodiments may have the ratio locked into alloy
particles of IIB-IVA, IB-IIB, IB-IVA or IB-IIB-IVA. This IIB rich
composition may also be obtained by printing an additional layer of
IIB over any existing IB IIB IVA precursor.
[0063] Optionally, some embodiment may desire to lock in the IIB
material be using core-shell particles with IIB in the core and a
IB, IVA, or VIA shell. Optionally, some embodiments may use a IB,
IVA, or VIA layer deposited over a precursor layer of IB IIB
IVA.
[0064] For the present embodiment, an approximately 0.5-2.5 micron
thick layer of the precursor material containing solution is
deposited on the substrate. The precursor material may be dispersed
in a solvent such as water, alcohol or ethylene glycol with the aid
of organic surfactants and/or dispersing agents described herein to
form an ink.
[0065] The precursor layer is annealed with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225.degree. to about 575.degree. C. preferably
for about 30 seconds to about 600 seconds to enhance densification
and/or alloying between Cu, In, and Ga in an atmosphere containing
hydrogen or nitrogen gas, where the plateau temperature not
necessarily is kept constant in time. Some embodiments may heat to
a temperature of at least 500.degree. C. Optionally, some
embodiments may heat to a temperature of at least 505.degree. C.
Optionally, some embodiments may heat to a temperature of at least
510.degree. C. Optionally, some embodiments may heat to a
temperature of at least 515.degree. C. Optionally, some embodiments
may heat to a temperature of at least 520.degree. C. Optionally,
some embodiments may heat to a temperature of at least 525.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 530.degree. C. Optionally, some embodiments may heat to a
temperature of at least 535.degree. C. Optionally, some embodiments
may heat to a temperature of at least 540.degree. C. Optionally,
some embodiments may heat to a temperature of at least 545.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 550.degree. C.
[0066] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Some
embodiments may heat to a temperature of at least 500.degree. C.
Optionally, some embodiments may heat to a temperature of at least
505.degree. C. Optionally, some embodiments may heat to a
temperature of at least 510.degree. C. Optionally, some embodiments
may heat to a temperature of at least 515.degree. C. Optionally,
some embodiments may heat to a temperature of at least 520.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 525.degree. C. Optionally, some embodiments may heat to a
temperature of at least 530.degree. C. Optionally, some embodiments
may heat to a temperature of at least 535.degree. C. Optionally,
some embodiments may heat to a temperature of at least 540.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 545.degree. C. Optionally, some embodiments may heat to a
temperature of at least 550.degree. C.
[0067] Optionally, instead of this two-step approach, the layer of
precursor material may be selenized without the separate annealing
step in an atmosphere containing hydrogen or nitrogen gas, but may
be densified and selenized in one step with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of 225 to 600.degree. C. for a time period of about 120
seconds to about 20 minutes in an atmosphere containing either H2Se
or a mixture of H2 and Se vapor. It should be understood that other
embodiments may be configured to include S vapor or H2S to create
the desired CIGS or CIGSS absorber.
[0068] The parts were finished into devices using a CdS junction
partner, a ZnO-based transparent conductor and a metallic grid top
contact. For the window layer, some embodiments may use ITO, B:ZnO,
A:ZnoY, non-vacuum deposited transparent conductors or other
transparent oxides.
Embodiment 4
[0069] In this embodiment, an extra layer of added group IB
material such as but not limited to silver or gold are added on top
of annealed precursor material or in the same layer during
printing. Optionally, the precursor layer comprises three or more
portions. These layers can be deposited by known techniques such as
electroplating, nanoparticle deposition, coevaporation, deposition
by gas, vacuum, or vapor phase techniques (vacuum evaporation,
sputtering, vapor transport, etc.) of the constituent elements
either in sequence or the simultaneous deposition of two or more
elements, or any combination thereof, or the like.
[0070] The first portion is next to the contact layer and it
comprises a metallic film having metallic Cu, In and optionally Ga
and/or Ag. This is the portion of the precursor where most of the
In and Cu are supplied from. The ingredients within the first
portion are metallic, either elemental or alloy form of Cu, In and
optionally Ga and/or Ag so that during the reaction step ACIGS film
can grow with good microstructure and large grains. Accordingly the
first portion may comprise stacks containing Ag, Cu, In and Ga (for
example Ag/Cu/In/Ga, Ag/Cu/Ga/In, etc. stacks) or stacks of
metallic alloys and elements (such as Cu--In/Cu--Ga, Cu--In/Ga,
Cu--Ga/In, Cu--Ga/Cu--In, Ag--In, Ag--Ga, etc.). The first portion
may be deposited by various techniques such as evaporation,
sputtering, ink or slurry deposition etc., however, preferred
method is electroplating. The thickness of the first portion may be
in the range of 400-2000 nm, preferably in the range of 500-1000
nm. The (Ag,Cu)/(In+Ga) molar ratio in the first portion may be in
the range of 0.7-1.2, preferably in the range of 0.8-1.0. In this
equation Ag, Cu, In and Ga represent the number of moles of Ag, Cu,
In and Ga, respectively, within the first portion. In one
embodiment, the ratio of Ag/IB=0.1-0.4. The In/D1 molar ratio may
be in the range of 0.25-0.6, where D1 represents the total number
of moles of all elements within the first portion, i.e. D1
comprises total number of moles of Ag, Cu, In, Ga and an additive
material such as a dopant material including one of Na, K, Li and
the like that may be present in the first portion. It should be
noted that there is no Group VIA material such as Se present in the
first portion.
[0071] The second portion or separator layer substantially
comprises a Group VIA material such as Se and Te with (VIA)2/D2
molar ratio being in the range of 0.95-1.0. In this equation (VIA)2
represents the number of moles of Group VIA materials and D2
represents the total number of moles of all elements within the
second portion. In other words, the second portion is substantially
made of a Group VIA material such as Se and Te, but it also may
include up to about 5% mole of other elements or additive materials
such as at least one of Ag, Cu, In, Ga, and a dopant material
including one of Na, K, Li and the like. Preferably, the second
portion comprises only a Group VIA material. The Group VIA material
may be Se or Te or a mixture of Se and Te. The thickness of the
second portion may be in the range of 50-1500 nm, preferably in the
range of 100-1000 nm. Various approaches such as evaporation and
ink or slurry deposition may be used to deposit the second portion,
but the preferred method is electroplating.
[0072] A third portion or source layer is formed on the second
portion. The third portion comprises Ga. For example, the third
portion may be a film of Ga, or it may additionally contain small
amounts of 1n and/or Cu. In any case the Ga3/D3 molar ratio within
the third portion may be in the range of 0.8-1.0, where Ga3
represents the number of moles of Ga and D3 represents the total
number of moles of all elements such as Cu and In that may be
present within the third portion. The third portion comprises
mostly Ga and additive materials such as metallic elements of Cu
and In and possibly dopants including one of Na, K, Li and the
like. The thickness of the third portion may be in the range of
10-200 microns, preferably in the range of 20-100 nm. The third
portion may be deposited using various thin film deposition
methods, however, the preferred method is evaporation.
[0073] The fourth portion or cap layer of the precursor layer
consists substantially of Se. This layer may be deposited using
various techniques but the preferred method is evaporation. The
thickness of the fourth portion may be in the range of 500-5000 nm,
preferably in the range of 800-3000 nm. The Se4/D4 molar ratio
within the fourth portion may be in the range of 0.95-1.0, where
Se4 represents the number of moles of Se and D4 represents the
total number of moles of all elements within the fourth portion. In
other words D4 includes other elements or additive materials that
may be present in the fourth portion such as Te and alkali metal
dopant materials including one of Na, K, Li and the like.
[0074] Each portion described above has a function within the
unique structure of the precursor layer of the present invention.
The first portion is the source or provider of most of the Ag, Cu
and In, and optionally Ga of the overall precursor layer. The
second portion is a separator between the first portion and the
third portion and it provides a Group VIA material such as Se or Te
to both the first portion and the Ga-containing third portion when
the temperature of the precursor layer is rapidly raised above
400.degree. C. Such reaction of Se and/or Te with the Ga within the
third portion helps arrest Ga diffusion towards the contact layer
31 and keeps Ga close to the surface of the absorber after the
reaction step. It should be noted that even if Te is included in
the second portion, the absorber film obtained after the heating
and reaction of the precursor layer 32 would be substantially a
CIGS layer since the thickness of the second portion is much
smaller than that of the fourth portion which comprises mostly Se.
Since Te is a Group VIA material like Se and since the bandgap of
(Ag,Cu)InTe2 is very similar to the bandgap of (Ag,Cu)InSe2,
inclusion of some Te in the CIGS layer does not negatively impact
the quality of the resulting absorber layer.
[0075] Known methods to form ACIGS absorbers employed a (Au, Ag,
Cu)/In/Ga/Se precursor structure and rapid thermal processing to
convert this precursor structure into a CIGS absorber. Resulting
CIGS absorbers comprised segregated In-rich and Ga-rich sections
even though Ga was placed far away from the contact layer. The
reason for this is the fact that when the temperature of such a
precursor layer or stack is raised, Ga can react with the Se layer
placed on top of it as well as with the In layer and the Cu layer
placed under it. Gallium reaction and intermixing with In and Cu is
faster than its reaction with Se. Therefore, it in effect moves
towards the contact layer while In and Cu move towards the surface.
In the present invention Group VIA-rich second portion is placed
under the Ga-containing third portion so that this problem can be
avoided and the Group VIA-rich second portion acts as a barrier
between the In-containing first portion and the Ga-containing third
portion. If the metallic Ga of the third portion were to be placed
directly on top of the Cu and In containing first portion, without
placing the second portion between the two portions, metallic Ga of
the third portion would easily mix in with the metallic Cu and In
of the first portion and move towards the contact layer as
explained above.
[0076] The fourth portion provides the excess Se to the overall
compound absorber formation process and at the same time, since it
is in physical contact with the Ga-containing third portion, it
assists in reacting metallic Ga with Se and thus arrest its
diffusion from the surface region of the absorber during the
reaction. As the above discussion demonstrates, sandwiching the
Ga-containing third portion between Se and/or Te containing second
portion and Se containing fourth portion helps keep Ga near the
surface. Metallic Ag, Cu and In containing first portion is
relatively thick and it assists in forming a ACIGS layer with a
good microstructure and with large grains after the reaction. It
should be noted that all portions of the precursor layer are formed
at relatively low temperatures, typically below 100.degree. C.,
preferably below 50.degree. C. This way, substantially discrete
nature of each of the first portion, the second portion, the third
portion and the fourth portion is preserved without much reaction
between each portion. The reaction step is then carried out,
preferably in a different apparatus, using RTP approach as will be
described next. This is different from some prior art methods where
various species of Ag, Cu, In, Ga and Se are deposited at various
stages of the process on a heated substrate causing reaction and
compound absorber formation during the film deposition step.
[0077] In the present invention, the above mentioned Na effect may
be used to ones benefit. In that respect, instead of on the contact
layer, Na may be included in at least one of the second portion,
the third portion and the fourth portion of the precursor layer.
This can be achieved by depositing a discrete film (not shown) of a
Na-containing material (such as Na, Na--Se, Na--S, Na--F, Na--I,
etc.) within the stack defined by the second portion, the third
portion and the fourth portion.
[0078] Alternately a Na-containing material may be co-deposited
along with at least one of the second portion, the third portion
and the fourth portion. In any case, the equivalent thickness of
this Na-containing film may be in the range of 2-40 nm, preferably
in the range of 5-20 nm. By including Na in at least one of the Ga
and Se-rich portions, i.e. second, third and fourth portions, of
the overall precursor layer, diffusion of the Ga species (which
react with Se) down towards the contact layer is reduced because of
presence of Na within these Ga and Se-rich portions, and because of
the above mentioned nature of Na retarding inter-diffusion of
Ga-rich and In-rich phases. It should be noted that inclusion of Na
in the first portion is optional in this case. It should also be
noted that although Na is the preferred doping material, it may be
wholly or partially replaced by another alkali metal such as K and
Li.
[0079] It should be understood that a fifth portion or source layer
may be provided. The fifth portion comprises Au, Ag, or their
alloys. For example and not limitation, the fifth portion may be a
film of Ag, or it may additionally contain small amounts of 1n, Ga,
and/or Cu. In any case the Ag5/D5 molar ratio within the fifth
portion may be in the range of 0.8-1.0, where Ag represents the
number of moles of Ag and D3 represents the total number of moles
of all elements such as Ag, Cu, Ga, and In that may be present
within the fifth portion. The fifth portion comprises mostly Ga and
additive materials such as metallic elements of Cu, Ga, and In and
possibly dopants including one of Na, K, Li and the like. The
thickness of the fifth portion may be in the range of 10-200
microns, preferably in the range of 20-100 nm. The fifth portion
may be deposited using various thin film deposition methods,
however, the preferred method is evaporation.
[0080] It should be understood that the group IB material in the
fifth portion may be positioned between the first and second
portions, between the second and third portions, between the third
and fourth portions, and/or above the fourth portion. This fifth
portion is provided so that it can be an additive that minimizes
migration or phase separation of Ga to the back of the final
semiconductor layer. Some embodiments may co-deposit these layers
with any of the foregoing portions.
[0081] The above precursor layer may be processed by any of the
techniques mentioned above with the other embodiments.
[0082] When the fifth portion is deposited as the top most layer,
this may be done after the underlying layer has first been first
annealed and/or selenized/sulfadized before this layer is applied
and then reacted again with group VIA material. Subsequently, this
layer is heated with a ramp-rate of 1-5.degree. C./sec, preferably
over 5.degree. C./sec, to a temperature of about 225 to 600.degree.
C. for a time period of about 60 seconds to about 10 minutes in Se
vapor in a non-vacuum, where the plateau temperature not
necessarily is kept constant in time, to form the thin-film
containing one or more chalcogenide compounds containing Ag, Cu,
In, Ga, and Se.
Embodiment 5
[0083] In this embodiment, an electroplating technique may be used
to deposit one or more of the layer. The substrate may be an
insulating sheet or foil such as glass or polyimide or a conductive
sheet or foil such as stainless steel or aluminum alloy web. The
contact layer may comprise a conducting material such as Mo, Ta,
Ru, Ir and Os among others. The precursor layer is grown on the top
surface of the contact layer. A Mo coated substrate may be used as
the substrate. An approximately 100 nm thick Ag and/or Cu layer may
be electrodeposited over the Mo layer. In one embodiment, the ratio
of Ag/IB=0.1-0.4. In one embodiment, the ratio of Ag/IB=0.25-0.35.
This is then followed by the electrodeposition of an about 220 nm
thick In film and a nominally 40 nm thick Ga layer. The stack is
annealed at a temperature of 80-200.degree. C. preferably for 2-300
seconds to enhance alloying between Ag, Cu, In and Ga. Over the
alloyed layer, a nominally 100 nm of Cu, a nominally 220 nm of 1n
and about 40 nm of Ga are then electrodeposited. A second anneal
step is applied at 80-200.degree. C. preferably for 2-300 seconds
to promote further alloying between the layers of the metallic
precursor. The precursor thus obtained is then selenized by well
known approaches such as in hydrogen selenide or selenium vapor for
a time period of 5 minutes to 60 minutes to form the
Ag--Cu--In--Ga--Se compound. It should be noted that selenization
may be carried out by various other means such as depositing Se
over the metallic precursor and heating up the stacked layer,
heating the substrate in a Se-containing gaseous or liquid
atmosphere etc.
[0084] It is believed that when a substantially metallic precursor
film including metallic Ag, Au, Cu, In and Ga is deposited first on
a contact layer (such as Mo) of a base and then reacted with Se,
the Ga-rich phases segregate to the film/base interface (or the
film/contact layer interface) because reactions between Ga-bearing
species and Se are slower than the reactions between In-bearing
species and Se. Results suggest that presence of Na reduces
inter-diffusion between the In-rich and Ga-rich phases and promotes
segregation of 1n-rich and Ga-rich phases through the thickness of
the absorber layer. On one hand an alkali metal such as Na is
desired to lower the resistivity of the resulting compound layers.
Additionally, NA may reduce the number of recombination centers to
create higher quality material, with longer minority-carrier
lifetimes. Therefore Na is usually included in the precursor layers
including Ag, Cu, In and Ga by deposition of a Na compound at the
contact layer/precursor layer interface, but on the other hand,
presence of Na encourages the formation of non-uniform Ga
distribution.
[0085] Optionally, a Na-containing material may be co-deposited
along with at least one of the second portion, the third portion
and the fourth portion. In any case, the equivalent thickness of
this Na-containing film may be in the range of 2-40 nm, preferably
in the range of 5-20 nm. By including Na in at least one of the Ga
and Se-rich portions, i.e. second, third and fourth portions, of
the overall precursor layer, diffusion of the Ga species (which
react with Se) down towards the contact layer is reduced because of
presence of Na within these Ga and Se-rich portions, and because of
the above mentioned nature of Na retarding inter-diffusion of
Ga-rich and In-rich phases. It should be noted that inclusion of Na
in the first portion 32A is optional in this case. It should also
be noted that although Na is the preferred doping material, it may
be wholly or partially replaced by another alkali metal such as K
and Li.
[0086] Optionally, reaction of Cu, In and Se to form CuInSe2 may
start at around 300.degree. C., and therefore this reaction is
favorable compared to the reaction of Ga species which takes place
at higher temperatures typically above 500.degree. C. In other
words if a metallic precursor layer containing Cu, In, and Ga is
reacted with Se by increasing the temperature from room temperature
to 550.degree. C. at a slow rate, as the precursor is heated up to
around 300.degree. C. and beyond, Cu and In within the metallic
precursor would start forming CuInSe2 because both Cu and In would
easily diffuse to the surface and react with Se. As an example, let
us assume that a heating rate of 5.degree. C./sec is employed
during a reaction process. This means that it would take
(550-300)/5=50 seconds for the temperature of the precursor to go
from 300.degree. C. to 550.degree. C. During this long period a
large percentage of the In within the metallic precursor is
expected to react with Cu and Se to form a surface layer which is
rich in In. This would be true even if the precursor layer
comprises a Ga-rich or pure Ga surface, such as a Cu/In/Ga
precursor stack deposited in that order over a base including a
substrate and a contact layer. Since reaction of Ga species takes
place at higher temperatures (typically higher than 500.degree.
C.), it is important to increase the rate of temperature rise,
especially beyond 300.degree. C. Use of a temperature rise rate of
10.degree. C./sec would reduce the reaction time before the
formation of Ga-rich phase to about 25 seconds. For the special
precursor structure of the present invention this may be adequate
since Ga is confined or sandwiched between two Group VIA-rich
layers (the second portion 32B and the fourth portion 32D),
however, the temperature rise rate for temperatures in the range of
300-600.degree. C. is preferably higher than 20.degree. C./sec,
more preferably higher than 30.degree. C./sec, most preferably
higher than about 5.degree. C./sec. For a rate of 50.degree.
C./sec, the temperature of the precursor is expected to go from
300.degree. C. to 550.degree. C. in 5 seconds. This would help the
Ga species reaction kinetics to catch up with that of 1n species
since the temperature goes above 500.degree. C. in 4 seconds and Ga
species can also start reacting along with In species. The special
structure of the precursor layer 32 of the present invention also
increases the residence time of Ga species near the surface and
helps provide an absorber layer with increased Ga content at its
surface.
[0087] In an exemplary CIGS absorber layer, the layer is formed by
heating the structure to a temperature above 500.degree. C. in an
inert atmosphere or in an atmosphere containing Se. The heating
rate for the temperature range between 300.degree. C. and the
highest temperature (which may be in the 550-600.degree. C. range)
is preferably higher than 20.degree. C./sec, more preferably higher
than 30.degree. C./sec, most preferably higher than 50.degree.
C./sec. As can be seen from this figure the CIGS absorber layer 40
comprises a surface region, wherein the surface region comprises a
Ga/(Ga+In) ratio in the range of 0.1-0.3. The thickness of the
surface region is in the range of 0.1-0.5 um, preferably in the
range of 0.1-0.3 um, whereas the total thickness of the CIGS
absorber layer 40 may be 0.8-3.0 um, preferably 1-2 um. Below the
surface region, the Ga/(Ga+In) ratio within the bulk of the
absorber layer 40 depends on the composition of the first portion
of the precursor layer. Depending upon the Ga content of the first
portion, (Ga+In) ratio within the bulk of the absorber may change
between 0 and 0.8, preferably between 0.1 and 0.6.
[0088] With these thin film stacks, copper layers (or In layers)
may be electroplated or sputter deposited on a base comprising a
substrate which, on its surface may have a conductive contact film
such as a Mo layer and/or a Ru-containing layer. The substrate may
be a metallic foil, glass or polymeric sheet or web. The Ru
containing layer on the substrate surface may be a Ru layer, a
Ru-alloy layer, a Ru compound layer or a stack containing Ru such
as a Mo/Ru stack or in general a N/Ru stack, where M is a conductor
or semiconductor. Gallium electroplating on the Cu surface (or the
In surface) can be carried out at various current densities, such
as at 5, 10, 20, 30, 40 and 50 mA/cm.sup.2, using the electrolytes
of the present invention. Both DC and/or variable (such as pulsed
or ramped) voltage/current waveforms may be used for electroplating
the Ga layer.
[0089] A set of exemplary aqueous plating baths were prepared
containing 0.2-0.5 M GaCl3, and 0.5-0.8 M sodium citrate
(Na3C6H5O7). The pH was adjusted to a range between 10 and 13.
Gallium was electrodeposited on the copper surface at current
densities of 30-50 mA/cm.sup.2. Highly adherent Ga films with
surface roughness of <10 nm were obtained for a thickness of 100
nm. The plating efficiency was measured and found to be in the
85-100% range, the higher current density yielding more efficient
deposition process. Gallium was also plated on other metal surfaces
also using the citrate containing complexed baths. Deposition on Ru
surface directly yielded a plating efficiency of 75-90%. On the
surface of 1n, Ga deposition efficiency reached 100%.
[0090] An aqueous plating bath was formulated with 0.2 M GaCl3 and
0.5 M Glycine. The pH was adjusted to the range of 11-13 using
NaOH. The plating tests were carried out on the surfaces of
electroplated copper at current densities of 10-50 mA/cm.sup.2. All
Ga films were shiny with smooth surfaces. Surface roughness was
<10 nm for 100 nm thick layers.
[0091] A Mo coated glass sheet may be used as the base. A 100 nm
thick Ag, Cu layer may be deposited over the Mo layer. In one
embodiment, the ratio of Ag/IB=0.1-0.4. This is then followed by
the deposition of a 220 nm thick In film and a 40 nm thick Ga
layer. The stack is annealed at a temperature of 80-200.degree. C.
for 5-600 seconds to enhance alloying between Ag, Cu, In and Ga.
Over the alloyed layer, 100 nm of Cu, 220 nm of 1n and 40 nm of Ga
are then deposited or applied. The precursor is selenized by well
known approaches such as in hydrogen selenide gas or selenium vapor
to form the (Ag,Cu)--In--Ga--Se compound. It should be noted that
selenization may be carried out by various other means such as
depositing Se over the metallic precursor and heating up the
stacked layer, heating the substrate in a Se-containing gaseous or
liquid atmosphere etc. for times ranging from 5 minutes to 60
minutes.
[0092] A Mo coated metal foil may be used as the base. A 100 nm
thick Ag, Cu layer may be deposited over the Mo layer. In one
embodiment, the ratio of Ag/IB=0.1-0.4. This is then followed by
the deposition of a 220 nm thick In film and a 40 nm thick Ga
layer. The stack is annealed at a temperature of 80-200.degree. C.
for 5-600 seconds to enhance alloying between Ag, Cu, In and Ga.
Over the alloyed layer, a 100 nm of Cu, a 220 nm of 1n and 40 nm of
Ga are then deposited. A second anneal step is applied at
80-200.degree. C. for 5-600 seconds to promote further alloying
between the layers of the metallic precursor. The precursor thus
obtained is then selenized by well known approaches such as in
hydrogen selenide or selenium vapor to form the (Ag,Cu)--In--Ga--Se
compound. It should be noted that selenization may be carried out
by various other means such as depositing Se over the metallic
precursor and heating up the stacked layer, heating the substrate
in a Se-containing gaseous or liquid atmosphere etc. for times
ranging from 5 minutes to 60 minutes.
[0093] In this embodiment, the approaches in the previous two
paragraphs are used except that Ag, Cu, In and Ga layers may be
deposited in four steps instead of two steps. Accordingly,
thickness of (Ag, Cu), In and Ga for each deposition step may be
reduced to 50 nm, 110 nm and 20 nm, respectively. For any of the
embodiments herein, the Ag, Cu layer may be co deposited or in
separate steps. By heat treating the layers after each deposition
step for reduced times of preferably 2-300 seconds (except for the
last one for the case of Example 1), a smooth and compositionally
uniform metallic precursor may be obtained. Selenization of this
precursor yields compositionally uniform, high quality
(Ag,Cu)--In--Ga--Se compound layer.
Embodiment 6
[0094] In yet another nonlimiting embodiment, Ag was introduced
into the precursor ink of Group IB-IIIA mixture (as described
herein) in dispersion form. By way of example and not limitation,
the Ag particles used were nanoparticle flakes with roughly 500 nm
in diameter. The particles were dispersed in an ink at a solids
loading of 15-20 wt % with a surfactant. This dispersion was
sonicated. The particle size distribution (PSD) of this dispersion
was found to have a median size of 500-700 nm and a d90 of 800-1100
nm. Coatings of this dispersion were made and looked at under SEM
which confirmed the size <500 nm and flake shape.
[0095] Cu-, In-, Ga- and Na-containing particle dispersions were
also prepared. These were added together with the Ag particle
dispersion to produce a desired composition with ratios
(Ag+Cu)/(In+Ga)=0.84, Ga/(In+Ga)=0.4 to 0.47, Ag/(Ag+Cu)=0.10. It
should be understood that any of the embodiments herein may include
Na/(Ag+Cu+In+Ga) of 0.01 to 0.03. Substrates were then coated with
this ink and some fine coating defects were visible, as well as a
moderate amount of large fisheye coating defects in these coatings,
but the coatings were glossy.
[0096] The substrates were then annealed at 520.degree. C. The
resulting annealed ACIG looked very poor, not because of darkness
of under-annealing, or non-uniform annealing, but because of a
cellular patterning visible by eye. Under optical microscope and
SEM this patterning was revealed to be rings of Mo exposure
surrounding 100 .mu.m sized areas that more or less appeared to be
normal structure, but were somewhat impacted by the addition of
Ag.
[0097] Despite the poor appearance of the anneal morphology, they
were selenized using one or more techniques for introducing group
VIA material. The parts suffered no delamination and appeared
fairly smooth and shiny. Both eye and SEM confirmed that no
patterning remained after RTP, indicating that these 100 um scale
non-uniformities were healed by selenization. Calculated E.sub.g
was found to be as high as 1.2 eV.
[0098] The parts were finished into devices using a CdS junction
partner, a ZnO-based transparent conductor and a metallic grid top
contact. The resulting solar cells had efficiencies above 12%, and
devices were created with open-circuit voltages above 660 mV.
Embodiment 7
[0099] A metal foil substrate was sputtered with Mo to form a 500
nm to 1000 nm Mo film to serve as the back contact. A barrier layer
of 50 nm to 300 nm of Chromium, TiN, HfN, or other transition metal
nitride barrier was formed onto the Mo back contact. A precursor
ink comprised of Cu, In, Ga, and Ag, and atomic ratios
Ag/(Ag+Cu)=0.1, Ga/(Ga+In)=0.4, and (Ag+Cu)/(Ga+In)=0.82. For the
present embodiment, an approximately 0.5-2.5 micron thick layer of
the precursor material containing solution is deposited on the
substrate. The precursor material may be dispersed in a solvent
such as water, alcohol or ethylene glycol with the aid of organic
surfactants and/or dispersing agents described herein to form an
ink.
[0100] The precursor layer is annealed with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225.degree. to about 575.degree. C. preferably
for about 30 seconds to about 600 seconds to enhance densification
and/or alloying between Cu, In, and Ga in an atmosphere containing
hydrogen or nitrogen gas, where the plateau temperature not
necessarily is kept constant in time. Some embodiments may heat to
a temperature of at least 500.degree. C. Optionally, some
embodiments may heat to a temperature of at least 505.degree. C.
Optionally, some embodiments may heat to a temperature of at least
510.degree. C. Optionally, some embodiments may heat to a
temperature of at least 515.degree. C. Optionally, some embodiments
may heat to a temperature of at least 520.degree. C. Optionally,
some embodiments may heat to a temperature of at least 525.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 530.degree. C. Optionally, some embodiments may heat to a
temperature of at least 535.degree. C. Optionally, some embodiments
may heat to a temperature of at least 540.degree. C. Optionally,
some embodiments may heat to a temperature of at least 545.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 550.degree. C.
[0101] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Some
embodiments may heat to a temperature of at least 500.degree. C.
Optionally, some embodiments may heat to a temperature of at least
505.degree. C. Optionally, some embodiments may heat to a
temperature of at least 510.degree. C. Optionally, some embodiments
may heat to a temperature of at least 515.degree. C. Optionally,
some embodiments may heat to a temperature of at least 520.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 525.degree. C. Optionally, some embodiments may heat to a
temperature of at least 530.degree. C. Optionally, some embodiments
may heat to a temperature of at least 535.degree. C. Optionally,
some embodiments may heat to a temperature of at least 540.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 545.degree. C. Optionally, some embodiments may heat to a
temperature of at least 550.degree. C.
[0102] Optionally, instead of this two-step approach, the layer of
precursor material may be selenized without the separate annealing
step in an atmosphere containing hydrogen or nitrogen gas, but may
be densified and selenized in one step with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of 225 to 600.degree. C. for a time period of about 120
seconds to about 20 minutes in an atmosphere containing either H2Se
or a mixture of H2 and Se vapor. It should be understood that other
embodiments may be configured to include S vapor or H2S to create
the desired CIGS or CIGSS absorber.
[0103] These absorber layer films were then used in the fabrication
of photovoltaic devices by chemical bath deposition of 50 nm CdS,
followed by sputter deposition of a 200 nm ZnO:ITO (indium tin
oxide) window layer, followed by e-beam deposition of a Ni--Al grid
structure.
Embodiment 8
[0104] A metal foil substrate was sputtered with Mo to form a 500
nm to 1000 nm Mo film to serve as the back contact. A barrier layer
of 50 nm to 300 nm of Chromium, TiN, HfN, or other transition metal
nitride barrier was formed onto the Mo back contact. A precursor
ink comprised of Cu, In, Ga, and AgIn2, and atomic ratios
Ag/(Ag+Cu)=0.05-0.15, Ga/(Ga+In)=0.4-0.5, and (Ag+Cu)/(Ga+In)=0.82.
The material may be in the form nanoparticles and/or flake
particles. For the present embodiment, an approximately 0.5-2.5
micron thick layer of the precursor material containing solution is
deposited on the substrate. The precursor material may be dispersed
in a solvent such as water, alcohol or ethylene glycol with the aid
of organic surfactants and/or dispersing agents described herein to
form an ink.
[0105] The precursor layer is annealed with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225.degree. to about 575.degree. C. preferably
for about 30 seconds to about 600 seconds to enhance densification
and/or alloying between Cu, In, and Ga in an atmosphere containing
hydrogen or nitrogen gas, where the plateau temperature not
necessarily is kept constant in time. Some embodiments may heat to
a temperature of at least 500.degree. C. Optionally, some
embodiments may heat to a temperature of at least 505.degree. C.
Optionally, some embodiments may heat to a temperature of at least
510.degree. C. Optionally, some embodiments may heat to a
temperature of at least 515.degree. C. Optionally, some embodiments
may heat to a temperature of at least 520.degree. C. Optionally,
some embodiments may heat to a temperature of at least 525.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 530.degree. C. Optionally, some embodiments may heat to a
temperature of at least 535.degree. C. Optionally, some embodiments
may heat to a temperature of at least 540.degree. C. Optionally,
some embodiments may heat to a temperature of at least 545.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 550.degree. C.
[0106] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Some
embodiments may heat to a temperature of at least 500.degree. C.
Optionally, some embodiments may heat to a temperature of at least
505.degree. C. Optionally, some embodiments may heat to a
temperature of at least 510.degree. C. Optionally, some embodiments
may heat to a temperature of at least 515.degree. C. Optionally,
some embodiments may heat to a temperature of at least 520.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 525.degree. C. Optionally, some embodiments may heat to a
temperature of at least 530.degree. C. Optionally, some embodiments
may heat to a temperature of at least 535.degree. C. Optionally,
some embodiments may heat to a temperature of at least 540.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 545.degree. C. Optionally, some embodiments may heat to a
temperature of at least 550.degree. C.
[0107] Optionally, instead of this two-step approach, the layer of
precursor material may be selenized without the separate annealing
step in an atmosphere containing hydrogen or nitrogen gas, but may
be densified and selenized in one step with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of 225 to 600.degree. C. for a time period of about 120
seconds to about 20 minutes in an atmosphere containing either
H.sub.2Se or a mixture of H.sub.2 and Se vapor. It should be
understood that other embodiments may be configured to include S
vapor or H.sub.2S to create the desired CIGS or CIGSS absorber. By
way of non-limiting example, the S material may be introduced at a
lower processing, such as between 200 to 250.degree. C.
[0108] These absorber layer films were then used in the fabrication
of photovoltaic devices by chemical bath deposition of 50 nm CdS,
followed by sputter deposition of a 200 nm metal oxide window
layer, followed by e-beam deposition of an electrically conductive
grid structure.
Embodiment 9
[0109] A metal foil substrate was sputtered with Mo to form a 500
nm to 1000 nm Mo film to serve as the back contact. A barrier layer
of 50 nm to 300 nm of Chromium, TiN, HfN, or other transition metal
nitride barrier was formed onto the Mo back contact. A precursor
ink comprised of Cu, In, AgGa3, and AgIn2 or Ag, and atomic ratios
Ag/(Ag+Cu)=0.05-0.15, Ga/(Ga+In)=0.4-0.5, and (Ag+Cu)/(Ga+In)=0.82.
The material may be in the form nanoparticles and/or flake
particles. For the present embodiment, an approximately 0.5-2.5
micron thick layer of the precursor material containing solution is
deposited on the substrate. The precursor material may be dispersed
in a solvent such as water, alcohol or ethylene glycol with the aid
of organic surfactants and/or dispersing agents described herein to
form an ink.
[0110] The precursor layer is annealed with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of about 225.degree. to about 575.degree. C. preferably
for about 30 seconds to about 600 seconds to enhance densification
and/or alloying between Cu, In, and Ga in an atmosphere containing
hydrogen or nitrogen gas, where the plateau temperature not
necessarily is kept constant in time. Some embodiments may heat to
a temperature of at least 500.degree. C. Optionally, some
embodiments may heat to a temperature of at least 505.degree. C.
Optionally, some embodiments may heat to a temperature of at least
510.degree. C. Optionally, some embodiments may heat to a
temperature of at least 515.degree. C. Optionally, some embodiments
may heat to a temperature of at least 520.degree. C. Optionally,
some embodiments may heat to a temperature of at least 525.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 530.degree. C. Optionally, some embodiments may heat to a
temperature of at least 535.degree. C. Optionally, some embodiments
may heat to a temperature of at least 540.degree. C. Optionally,
some embodiments may heat to a temperature of at least 545.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 550.degree. C.
[0111] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Some
embodiments may heat to a temperature of at least 500.degree. C.
Optionally, some embodiments may heat to a temperature of at least
505.degree. C. Optionally, some embodiments may heat to a
temperature of at least 510.degree. C. Optionally, some embodiments
may heat to a temperature of at least 515.degree. C. Optionally,
some embodiments may heat to a temperature of at least 520.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 525.degree. C. Optionally, some embodiments may heat to a
temperature of at least 530.degree. C. Optionally, some embodiments
may heat to a temperature of at least 535.degree. C. Optionally,
some embodiments may heat to a temperature of at least 540.degree.
C. Optionally, some embodiments may heat to a temperature of at
least 545.degree. C. Optionally, some embodiments may heat to a
temperature of at least 550.degree. C.
[0112] Optionally, instead of this two-step approach, the layer of
precursor material may be selenized without the separate annealing
step in an atmosphere containing hydrogen or nitrogen gas, but may
be densified and selenized in one step with a ramp-rate of
1-5.degree. C./sec, preferably over 5.degree. C./sec, to a
temperature of 225 to 600.degree. C. for a time period of about 120
seconds to about 20 minutes in an atmosphere containing either
H.sub.2Se or a mixture of H.sub.2 and Se vapor. It should be
understood that other embodiments may be configured to include S
vapor or H.sub.2S to create the desired CIGS or CIGSS absorber. By
way of non-limiting example, the S material may be introduced at a
lower processing, such as between 200 to 250.degree. C.
[0113] These absorber layer films were then used in the fabrication
of photovoltaic devices by chemical bath deposition of 50 nm CdS,
followed by deposition of a 200 nm metal oxide window layer,
followed by e-beam deposition of an electrically conductive grid
structure.
Embodiment 10
[0114] In this embodiment, a highly phase segregated material is
used. The precursor material is provided that has areas of AgIn2
and/or Ag3Ga in the precursor material. This may be in the form of
dark areas 600 as seen in FIG. 6. Even though there may be patches
of these materials in precursor material prior to selenization
and/or sulfadation in this order or vice versa. A precursor
material comprised of Cu, In, Ga, and Ag, and atomic ratios
Ag/(Ag+Cu)=0.1-0.4, Ga/(Ga+In)=0.25-0.4, and
(Ag+Cu)/(Ga+In)=0.80-0.90. The substrate may be an insulating sheet
or foil such as glass or polyimide or a conductive sheet or foil
such as stainless steel or aluminum alloy web. The contact layer
may comprise a conducting material such as Mo, Ta, Ru, Ir and Os
among others. The precursor layer is grown on the top surface of
the contact layer. A Mo coated substrate may be used as the
substrate. The precursor thus obtained is then selenized by well
known approaches such as in hydrogen selenide or selenium vapor for
a time period of 5 minutes to 60 minutes to form the ACIGS
compound. It should be noted that selenization may be carried out
by various other means such as depositing Se over the metallic
precursor and heating up the stacked layer, heating the substrate
in a Se-containing gaseous or liquid atmosphere etc. The general
idea is that selenization and/or sulfadation fixes the anneal
segregation.
[0115] The two step approach is from particles. It is not obvious
that using silver would bring the gallium forward. Others use
co-evaporation to deposit all materials all at once, and they do
not see gallium segregation. A co-evaporation process is grown from
the bottom up and thus does not see this gallium segregation
phenomenon. CuIn selenizes much more quickly than CuGa. With Ag (or
Au in gold embodiments) binding to the In, the reaction of CuIn
with Se is slowed down that would otherwise force Ga to the back.
Ag--In has a more noble component that is bound to the indium to
influence the rate of chalcogenation. Optionally, some intermediate
phases may be formed that react quicker with Ga to prevent it from
being pushed to the back. Gold is more noble than Silver. Silver is
more noble than copper. Silver is harder to oxidize than copper.
Without being bound to any particular technique, it reacts more
slowly than copper and thus slows the reaction kinetics of CuIn
with Se which is favorable for the reaction of the gallium
compounds.
[0116] The two stage processes creates massively phase separated
material which are then cured during selenization. The annealed
film looked rough. Ra is good, Rz (peak to peak) was higher, but
overall average Ra was smoother and in the range of about 230 nm or
less. Optionally, overall average Ra was smoother and in the range
of about 250 nm or less. The material may have areas of molybdenum
exposure. Some may have material in the precursor that includes 75%
Ga, 25% In (Cu9InGa4), Cu9Ga4, and/or Cu9In4. The composition is
uniform after selenization. CIGS morphology is smoother.
[0117] Subsequently, this annealed layer is selenized with a
ramp-rate of 1-5.degree. C./sec, preferably over 5.degree. C./sec,
to a temperature of about 225 to 600.degree. C. for a time period
of about 60 seconds to about 10 minutes in Se vapor in a
non-vacuum, where the plateau temperature not necessarily is kept
constant in time, to form the thin-film containing one or more
chalcogenide compounds containing Cu, In, Ga, and Se. Instead of
this two-step approach, the layer of precursor material may be
selenized without the separate annealing step in an atmosphere
containing hydrogen or nitrogen gas, but may be densified and
selenized in one step with a ramp-rate of 1-5.degree. C./sec,
preferably over 5.degree. C./sec, to a temperature of 225 to
600.degree. C. for a time period of about 120 seconds to about 20
minutes in an atmosphere containing either H2Se or a mixture of H2
and Se vapor. It should be understood that other embodiments may be
configured to include S vapor or H2S to create the desired CIGS or
CIGSS absorber. In one embodiment, the semiconductor is formed can
be ACIGCh1.sub.aCh2.sub.b alloy, with Ch1 being a first chalcogen
(S, Se, or Te), Ch2 being a second chalcogen (S, Se, or Te). Most
embodiments, Ch1 is different from Ch2. Ch1 can be introduced in
one process and Ch2 introduced in a second process. Optionally, Ch1
and Ch2 are introduced in the same process step.
[0118] It should be understood that for any of the embodiments
herein, any combination of the following mixed with CIG or other
group IB-based precursor ink: core-shell particles (e.g. Ag--Cu or
Cu--Ag), Ag nanowires, Cu-coated Ag nanowires, Ag-coated Cu
nanowires, ALD of Ag-organometallic precusors, sputtered Ag,
evaporated Ag, sputtered Ag--Cu, evaporated Ag--Cu, and so forth.
Also included are other deposition techniques such as
electroplating of Ag onto CIGS and/or pressure-based printing of Ag
onto CIGS using pressure techniques such as described in
US20030052382 or US20070160763 both fully incorporated herein by
reference for all purposes. Combinations of any or all of these may
be used to minimize phase segregation (due to different extents of
exposed interfacial area available for chemical reaction, which
will be different for different deposition techniques). Although Ag
is described herein, it should be understood that Ag may be used in
combination with Au or replaced by Au completely in any of the
embodiments herein.
Photovoltaic Device Chemistry
[0119] The solid particles for use with the present invention may
be used with a variety of different chemistries to arrive at a
desired semiconductor film. Although not limited to the following,
an active layer for a photovoltaic device may be fabricated by
formulating an ink of spherical and/or non-spherical particles each
containing at least one element from groups IB, IIIA and/or VIA,
coating a substrate with the ink to form a precursor layer, and
heating the precursor layer to form a dense film. By way of
nonlimiting example, the particles themselves may be elemental
particles or alloy particles. In some embodiments, the precursor
layer forms the desired group IB-IIIA-VIA compound in a one step
process. In other embodiments, a two step process is used wherein a
dense film is formed and then further processed in a suitable
atmosphere to form the desired group IB-IIIA-VIA compound. It
should be understood that chemical reduction of the precursor layer
may not be needed in some embodiments, particularly if the
precursor materials are oxygen-free or substantially oxygen free.
Thus, a first heating step of two sequential heating steps may
optionally be skipped if the particles are processed air-free and
are oxygen-free.
[0120] It should also be understood that group IB, IIIA, and VIA
elements other than Cu, In, Ga, Se, and S may be included in the
description of the IB-IIIA-VIA materials described herein, and that
the use of a hyphen ("-" e.g., in Cu--Se or Cu--In--Se) does not
indicate a compound, but rather indicates a coexisting mixture of
the elements joined by the hyphen. It is also understood that group
IB is sometimes referred to as group 11, group IIIA is sometimes
referred to as group 13 and group VIA is sometimes referred to as
group 16. Furthermore, elements of group VIA (16) are sometimes
referred to as chalcogens. Where several elements can be combined
with or substituted for each other, such as In and Ga, or Se, and
S, in embodiments of the present invention, it is not uncommon in
this art to include in a set of parentheses those elements that can
be combined or interchanged, such as (In, Ga) or (Se, S). The
descriptions in this specification sometimes use this convenience.
Finally, also for convenience, the elements are discussed with
their commonly accepted chemical symbols. Group IB elements
suitable for use in the method of this invention include copper
(Cu), silver (Ag), and gold (Au). Preferably the group IB element
is copper (Cu). Optionally, the group IB element includes a) copper
(Cu) and b) silver (Ag) or gold (Au). Group IIIA elements suitable
for use in the method of this invention include gallium (Ga),
indium (In), aluminum (Al), and thallium (Tl). Preferably the group
IIIA element is gallium (Ga) and/or indium (In). Group VIA elements
of interest include selenium (Se), sulfur (S), and tellurium (Te),
and preferably the group VIA element is either Se and/or S. It
should be understood that mixtures such as, but not limited to,
alloys, solid solutions, and compounds of any of the above can also
be used. The shapes of the solid particles may be any of those
described herein.
Forming a Film from Particle Precursors
[0121] Referring now to FIGS. 1A-1D, one method of forming a
semiconductor film from particles of precursor materials according
to the present invention will now be described. It should be
understood that the present embodiment uses non-vacuum techniques
to form the semiconductor film. Other embodiments of the invention,
however, may optionally form the film under a vacuum environment,
and the use of solid particles (non-spherical and/or spherical) is
not limited to only non-vacuum deposition or coating techniques.
Optionally, some embodiments may combine both vacuum and non-vacuum
techniques.
[0122] As seen in FIG. 1A, a substrate 102 is provided on which the
precursor layer 106 (see FIG. 1B) will be formed. By way of
non-limiting example, the substrate 102 may be made of a metal such
as stainless steel or aluminum. In other embodiments, metals such
as, but not limited to, copper, steel, coated aluminum, molybdenum,
titanium, tin, metallized plastic films, or combinations of the
foregoing may be used as the substrate 102. Alternative substrates
include but are not limited to ceramics, glasses, and the like. Any
of these substrates may be in the form of foils, sheets, rolls, the
like, or combinations thereof. Depending on the conditions of the
surface, and material of the substrate, it may be useful to clean
and/or smoothen the substrate surface. Furthermore, depending on
the material of the substrate 102, it may be useful to coat a
surface of the substrate 102 with a contact layer 104 to promote
electrical contact between the substrate 102 and the absorber layer
that is to be formed on it, and/or to limit reactivity of the
substrate 102 in subsequent steps, and/or to promote higher quality
absorber growth. As a non-limiting example, when the substrate 102
is made of aluminum, the contact layer 104 may be but is not
limited to a single or multiple layer(s) of molybdenum (Mo),
tungsten (W), tantalum (Ta), binary and/or multinary alloys of Mo,
W, and/or Ta, with or without the incorporation of a IA element
like sodium, and/or oxygen, and/or nitrogen. Some embodiment may
include a contact layer 104 may be comprised of a molybdenium-IA
material such as but not limited to Na--Mo, Na--F--Mo, or the like
deposited using a vacuum or non-vacuum technique. For the purposes
of the present discussion, the contact layer 104 may be regarded as
being part of the substrate. As such, any discussion of forming or
disposing a material or layer of material on the substrate 102
includes disposing or forming such material or layer on the contact
layer 104, if one is used. Optionally, other layers of materials
may also be used with the contact layer 104 for insulation or other
purposes and still considered part of the substrate 102. It should
be understood that the contact layer 104 may comprise of more than
one type or more than one discrete layer of material. Optionally,
some embodiments may use any one and/or combinations of the
following for the contact layer: a copper, aluminum, chromium,
molybdenum, tungsten, tantalum, vanadium, etc. and/or iron-cobalt
alloys. Optionally, a diffusion barrier layer 103 (shown in
phantom) may be included and layer 103 may be electrically
conductive or electrically non-conductive. As non-limiting
examples, the layer 103 may be composed of any of a variety of
materials, including but not limited to chromium, vanadium,
tungsten, or compounds such as nitrides (including tantalum
nitride, tungsten nitride, titanium nitride, silicon nitride,
zirconium nitride, and/or hafnium nitride), oxy-nitrides (including
tantalum oxy nitride, tungsten oxy nitride, titanium oxy nitride,
silicon oxy nitride, zirconium oxy nitride, and/or hafnium oxy
nitride), oxides (including Al2O3 or SiO2), carbides (including
SiC), binary and/or multinary compounds of W, Ti, Mo, Cr, V, Ta,
Hf, Zr, and/or Nb, with/without the addition of either oxygen
and/or nitrogen into these elemental, binary and/or multinary
compound layers, and/or any single or multiple combination of the
foregoing. Optionally, a diffusion barrier layer 105 (shown in
phantom) may be on the underside of substrate 102 and be comprised
of a material such as but not limited to chromium, vanadium,
tungsten, or compounds such as nitrides (including tantalum
nitride, tungsten nitride, titanium nitride, silicon nitride,
zirconium nitride, and/or hafnium nitride), oxides (including
alumina, Al2O3, SiO2, or similar oxides), carbides (including SiC),
and/or any single or multiple combination of the foregoing. The
layers 103 and/or 105 may be adapted for use with any of the
embodiments described herein. The layer 105 may be the same or a
different material from that of layer 103.
[0123] Referring now to FIG. 1B, a precursor layer 106 is formed
over the substrate 102 by coating the substrate 102 with a
dispersion such as but not limited to an ink. As one non-limiting
example, the ink may be comprised of a carrier liquid mixed with
particles such as but not limited to flakes 108 and has a rheology
that allows the ink to be solution-deposited over the substrate
102. In one embodiment, the present invention may use a single dry
powder or a mixture of two or more dry powders mixed with the
vehicle containing or not containing a dispersant, and sonicated
before coating. Optionally, the inks may be already formulated as
the precursor materials are formed in a RF thermal plasma-based
size reduction chamber such that discussed in U.S. Pat. No.
5,486,675 fully incorporated herein by reference. Optionally, the
inks may be already formulated. In the case of mixing a plurality
of flake compositions, the product may be mixed from various
sources. This mixing could be by sonication but other forms of
mechanical agitation and/or a mill may also be used. The ink used
to form the precursor layer 106 may contain non-spherical particles
108 such as but not limited to microflakes and/or nanoflakes. It
should also be understood that the ink may optionally use both
non-spherical and spherical particles in any of a variety of
relative proportions.
[0124] FIG. 1B includes a close-up view of the particles in the
precursor layer 106, as seen in the enlarged image. Although not
limited to the following, the particles may be flakes 108 that have
non-spherical shapes and are substantially planar on at least one
side. A more detailed view of one embodiment of the flakes 108 can
be found in FIGS. 2A and 2B of U.S. patent application Ser. No.
11/362,266 filed Feb. 23, 2006 and fully incorporated herein by
reference. Microflakes may be defined as particles having at least
one substantially planar surface with a length and/or largest
lateral dimension of about 500 nm or more and the particles have an
aspect ratio of about 2 or more. In other embodiments, the
microflake is a substantially planar structure with thickness of
between about 10 and about 250 nm and lengths between about 500 nm
and about 5 microns. It should be understood that in other
embodiments of the invention, microflakes may have lengths of at
least 1 micron or more. It should be understood that in other
embodiments of the invention, microflakes may have lengths as large
as 10 microns. Although not limited to the following, at least some
of the solid group IIIA-particles may be processed into planar
particles and adapted for use during solution deposition.
[0125] In one non-limiting example, the particles used to form the
precursor layer 106 are elemental particles, i.e., having only a
single atomic species. In one embodiment, the ink used for
precursor layer 106 may contain particles comprising one or more
group IB elements and particles comprising one or more different
group MA elements. Preferably, the precursor layer 106 contains
copper, indium and gallium. In another embodiment, the precursor
layer 106 may be an oxygen-free layer containing copper, indium and
gallium. Optionally, the ratio of elements in the precursor layer
may be such that the layer, when processed, forms one or more
phases where the phases contain one or more of the elements Cu, In,
and Ga, and where the layer has the overall composition (Ag, Au,
Cu).sub.zIn.sub.xGa.sub.1-x, where 0.ltoreq.x.ltoreq.1 and
0.5.ltoreq.z.ltoreq.1.5.
[0126] Optionally, some of the particles in the ink may be alloy
particles. In one nonlimiting example, the particles may be binary
alloy particles such as but not limited to Ag--In, Ag--Ga, Au--In,
Au--Ga, Cu--In, In--Ga, or Cu--Ga. Alternatively, the particles may
be a binary alloy of group IB, IIIA elements, a binary alloy of
Group IB, VIA elements, and/or a binary alloy of group IIIA, VIA
elements. In other embodiments, the particles may be a ternary
alloy of group IB, IIIA, and/or VIA elements. For example, the
particles may be ternary alloy particles of any of the above
elements such as but not limited to Ag--In--Ga, Au--In--Ga,
Cu--In--Ga. In other embodiments, the ink may contain particles
that are a quaternary alloy of group IB, IIIA, and/or VIA elements.
Some embodiments may have quaternary or multi-nary particles. It
should also be understood that the source of group VIA material may
be added as discussed in commonly assigned, co-pending U.S. patent
application Ser. No. 11/243,522 (Attorney Docket No. NSL-046) filed
on Feb. 23, 2006 and fully incorporated herein by reference.
[0127] Generally, an ink may be formed by dispersing any of the
aforementioned particles (and/or other particles) in a vehicle
containing a dispersant (e.g., a surfactant or polymer) along with
(optionally) some combination of other components commonly used in
making inks In some embodiments of the present invention, the ink
is formulated without a dispersant or other additive. The carrier
liquid may be an aqueous (water-based) or non-aqueous (organic)
solvent. Other components include, without limitation, dispersing
agents, binders, emulsifiers, anti-foaming agents, dryers,
solvents, fillers, extenders, thickening agents, film conditioners,
anti-oxidants, flow and leveling agents, plasticizers and
preservatives. These components can be added in various
combinations to improve the film quality and optimize the coating
properties of the particle dispersion and/or improve the subsequent
densification. Any of the ink formulations, dispersants,
surfactants, or other additives described in U.S. patent
application Ser. No. 12/175,945 filed Jul. 18, 2008 and fully
incorporated herein by reference.
[0128] The precursor layer 106 from the dispersion may be formed on
the substrate 102 by any of a variety of solution-based coating
techniques including but not limited to wet coating, spray coating,
spin coating, doctor blade coating, contact printing, top feed
reverse printing, bottom feed reverse printing, nozzle feed reverse
printing, gravure printing, microgravure printing, reverse
microgravure printing, comma direct printing, roller coating, slot
die coating, meyerbar coating, lip direct coating, dual lip direct
coating, capillary coating, ink jet printing, jet deposition, spray
deposition, and the like, as well as combinations of the above
and/or related technologies. The foregoing may apply to any
embodiments herein, regardless of particle size or shape.
[0129] Note that the method may be optimized by using, prior to,
during, or after the solution deposition and/or (partial)
densification of one or more of the precursor layers, any
combination of (1) any (mixture of) chalcogen source(s) that can be
solution-deposited, e.g. a Se or S nanopowder mixed into the
precursor layers or deposited as a separate layer, (2) chalcogen
(e.g., Se or S) evaporation, (3) a (mixture of)
chalcogen-containing hydride gas(es) atmosphere (e.g. H.sub.2Se,
and/or H.sub.2S) at pressures below, equal to, and/or above
atmospheric pressure, (4) a steady-state and/or dynamic (mixture
of) chalcogen vapor(s) atmosphere (e.g., Se, and/or S) at pressures
below, equal to, and/or above atmospheric pressure, (5) an
organo-selenium containing atmosphere, e.g. diethylselenide, at
pressures below, equal to, and/or above atmospheric pressure, (6)
an H.sub.2 atmosphere at pressures below, equal to, and/or above
atmospheric pressure, (7) another reducing atmosphere, e.g. CO, (8)
a wet chemical reduction step, (9) generation of a plasma to break
the chemical bonds in the vapor(s) and/or gas(es) in the atmosphere
to increase the reactivity of these species, at pressures below,
equal to, and/or above atmospheric pressure, (10) a steady-state
and/or dynamic atmosphere containing a sodium source, (e.g. Na--Se
or Na--S), at pressures below, equal to, and/or above atmospheric
pressure, (11) liquid deposition of a chalcogen source, and a (12)
heat treatment.
[0130] Referring now to FIG. 1C, the precursor layer 106 of
particles may then be processed in a suitable atmosphere to form a
film. In one embodiment, this processing involves heating the
precursor layer 106 to a temperature sufficient to convert the ink
to a film (as-deposited ink; note that solvent and possibly
dispersant have been removed by drying or other removal technique).
The heating may involve various thermal processing techniques such
as pulsed thermal processing, exposure to laser beams, heating via
IR lamps, and/or similar or related processes. Although not limited
to the following, the temperature during heating may be between
about 375.degree. C. and about 525.degree. C. (a safe temperature
range for processing on aluminum foil or
high-temperature-compatible polymer substrates). The processing may
occur at various temperatures in this range, such as but not
limited to a constant temperature of 450.degree. C. In other
embodiments, the temperature may be between about 400.degree. C.
and about 600.degree. C. at the level of the precursor layer, but
cooler at the substrate. In other embodiments, the temperature may
be between about 500.degree. C. and about 600.degree. C. at the
level of the precursor layer.
[0131] The atmosphere associated with the annealing step in FIG. 1C
may also be varied. In one embodiment, the suitable atmosphere
comprises an atmosphere containing more than about 10% hydrogen. In
another embodiment the suitable atmosphere comprises a carbon
monoxide atmosphere. However, in other embodiments where very low
or no amounts of oxygen are found in the particles, the suitable
atmosphere may be a nitrogen atmosphere, an argon atmosphere, or an
atmosphere having less than about 10% hydrogen. These other
atmospheres may be advantageous to enable and improve material
handling during production.
[0132] Referring now to FIG. 1D, the precursor layer 106 processed
in FIG. 1C will form a film 110. The film 110 may actually have a
reduced thickness compared to the thickness of the wet precursor
layer 106 since the carrier liquid and other materials have been
removed during processing. In one nonlimiting embodiment, the film
110 may have a thickness in the range of about 0.5 microns to about
2.5 microns. In other embodiments, the thickness of film 110 may be
between about 1.5 microns and about 2.25 microns. In one
embodiment, the resulting dense film 110 may be substantially void
free. In some embodiments, the dense film 110 has a void volume of
about 5% or less. In other embodiments, the void volume is about
10% or less. In another embodiment, the void volume is about 20% or
less. In still other embodiments, the void volume is about 24% or
less. In still other embodiments, the void volume is about 30% or
less. The processing of the precursor layer 106 will fuse the
particles together and in most instances, remove void space and
thus reduce the thickness of the resulting dense film.
[0133] Depending on the type of materials used to form the film
110, the film 110 may be suitable for use as an absorber layer or
be further processed to become an absorber layer. More
specifically, the film 110 may be a film as a result of a one step
process, or for use in another subsequent one step process making
it a two step process, or for use in a multi-step process. In a one
step process, the film 110 is formed to include group IB-IIIA-VIA
compounds and the film 110 may be an absorber film suitable for use
in a photovoltaic device. In a two step process, the film 110 may
be a solid, annealed, and/or densified film that will have further
processing to be suitable for use as an absorber film for use in a
photovoltaic device. As a nonlimiting example, the film 110 in a
two step process may not contain any and/or sufficient amounts of a
group VIA element to function as an absorber layer. Adding a group
VIA element or other material may be the second step of the
two-step process. Either a mixture of two or more VIA elements can
be used, or a third step can be added with another VIA element as
used in the second step. A variety of methods of adding that
material include printing of group VIA element, using VIA element
vapor, and/or other techniques. It should also be understood that
in a two step process, the process atmospheres may be different. By
way of nonlimiting example, one atmosphere may optionally be a
group VIA-based atmosphere. As another nonlimiting example, one
atmosphere may be an inert atmosphere as described herein. Other
processing steps as used in a multi-step process may be a wet
chemical surface treatment to improve the IB-IIIA-VIA thin-film
surface and/or grain boundaries, and/or an additional rapid thermal
heating to improve bulk and/or surface properties of the
IB-IIIA-VIA thin-film.
Particle Shapes
[0134] It should be understood that any of solid particles as
discussed herein may be used in spherical and/or non-spherical
particle shapes. FIG. 1A shows that the particles may all be
non-spherical, planar flake particles. By way of example and not
limitation, it should be understood that the solid Group IIIA-based
particles may be particles of various shapes used with any of the
combinations shown below in Table III. Flakes may be considered to
be one type of non-spherical particles.
TABLE-US-00001 TABLE III Spherical Non-Spherical Flake Nanoglobules
Spherical Spherical Non-spherical + Flake + Nanoglobules +
Spherical Spherical Spherical Non-Spherical Spherical +
Non-spherical Flake + Nanoglobules + Non-spherical Non-spherical
Non-spherical Flake Spherical + Non-spherical + Flake Nanoglobules
+ Flake Flake Flake Nanoglobules Spherical + Non-spherical + Flake
+ Nanoglobules Nanoglobules Nanoglobules Nanoglobules Spherical +
Spherical + Spherical + Spherical + Spherical + Non-spherical
Non-spherical Non-spherical Non-spherical + Non-spherical + Flake
Nanoglobules Spherical + Spherical + Spherical + Spherical +
Spherical + Flake Flake Flake + Flake Flake + Non-spherical
Nanoglobules Spherical + Spherical + Spherical + Spherical +
Spherical + Nanoglobules Nanoglobules Nanoglobules + Nanoglobules +
Nanoglobules Non-spherical Flake Flake + Flake + Flake + Flake +
Flake + Nonspherical Nonspherical + Nonspherical Nonspherical
Nonspherical + Spherical Nanoglobules Flake + Flake + Flake + Flake
+ Flake + Nanoglobules Nanoglobules + Nanoglobules + Nanoglobules
Nanoglobules Spherical Non-spherical Non-spherical + Non-spherical
+ Non-spherical + Non-spherical + Non-spherical + Nanoglobules
Nanoglobules + Nanoglobules Nanoglobules + Nanoglobules Spherical
Flake
[0135] It should be understood that the salt particles described
herein may be size reduced to be spherical and/or non-spherical in
shape and is not limited to any one particular configuration.
Additional Sodium
[0136] Referring now to FIGS. 2A-2E, it should be understood that
even with solid group IIIA-based particles, more sodium may be
desired to provide improved performance. This embodiment of the
invention shows that layers of material may be deposited above
and/or below the precursor layer. Some layers may be deposited
after the precursor layer has been processed. Although not limited
to the following, these layers may provide one technique for adding
additional sodium.
[0137] Referring now to FIG. 2A, the absorber layer may be formed
on a substrate 312, as shown in FIG. 2A. A surface of the substrate
312 may be coated with a contact layer 314 to promote electrical
contact between the substrate 312 and the absorber layer that is to
be formed on it. By way of example, a metal substrate 312 may be
coated with a contact layer 314 of molybdenum. As discussed herein,
forming or disposing a material or layer of material on the
substrate 312 includes disposing or forming such material or layer
on the contact layer 314, if one is used. Optionally, it should
also be understood that a layer 315 may also be formed on top of
contact layer 314 and/or directly on substrate 312. This layer may
be solution coated, evaporated, and/or deposited using vacuum based
techniques. Although not limited to the following, the layer 315
may have a thickness less than that of the precursor layer 316. In
one nonlimiting example, the layer may be between about 1 nm to
about 100 nm in thickness. The layer 315 may be comprised of
various materials including but not limited to at least one of the
following: a group IB element, a group IIIA element, a group VIA
element, a group IA element (new style: group 1), a binary and/or
multinary alloy of any of the preceding elements, a solid solution
of any of the preceding elements, copper, indium, gallium,
selenium, copper indium, copper gallium, indium gallium, sodium, a
sodium compound, sodium fluoride, sodium indium sulfide, copper
selenide, copper sulfide, indium selenide, indium sulfide, gallium
selenide, gallium sulfide, copper indium selenide, copper indium
sulfide, copper gallium selenide, copper gallium sulfide, indium
gallium selenide, indium gallium sulfide, copper indium gallium
selenide, and/or copper indium gallium sulfide.
[0138] As shown in FIG. 2B, a precursor layer 316 is formed on the
substrate. The precursor layer 316 contains one or more group IB
elements and one or more group IIIA elements. Preferably, the one
or more group IB elements include copper. Optionally, the one or
more group IB elements include silver or gold. The one or more
group IIIA elements may include indium and/or gallium. The
precursor layer may be formed using any of the techniques described
above. In one embodiment, the precursor layer contains no oxygen
other than those unavoidably present as impurities or incidentally
present in components of the film other than the flakes themselves.
Although the precursor layer 316 is preferably formed using
non-vacuum methods, it should be understood that it may optionally
be formed by other means, such as evaporation, sputtering, chemical
vapor deposition, physical vapor deposition, atomic layer
deposition, ALD, etc. By way of example, the precursor layer 316
may be an oxygen-free compound containing copper, indium and
gallium. In one embodiment, the non-vacuum system operates at
pressures above about 3.2 kPa (24 Torr). Optionally, it should also
be understood that a layer 317 may also be formed on top of
precursor layer 316. It should be understood that the stack may
have both layers 315 and 317, only one of the layers, or none of
the layers. Although not limited to the following, the layer 317
may have a thickness less than that of the precursor layer 316. In
one nonlimiting example, the layer may be between about 1 to about
100 nm in thickness. The layer 317 may be comprised of various
materials including but not limited to at least one of the
following: a group IB element, a group IIIA element, a group VIA
element, a group IA element (new style: group 1), a binary and/or
multinary alloy of any of the preceding elements, a solid solution
of any of the preceding elements, copper, indium, gallium,
selenium, copper indium, copper gallium, indium gallium, sodium, a
sodium compound, sodium fluoride, sodium indium sulfide, copper
selenide, copper sulfide, indium selenide, indium sulfide, gallium
selenide, gallium sulfide, copper indium selenide, copper indium
sulfide, copper gallium selenide, copper gallium sulfide, indium
gallium selenide, indium gallium sulfide, copper indium gallium
selenide, and/or copper indium gallium sulfide.
[0139] Referring now to FIG. 2C, heat 320 is applied to densify the
first precursor layer 316 into a group IB-IIIA compound film 322.
The heat 320 may be supplied in a rapid thermal annealing process,
e.g., as described above. As a nonlimiting example, the substrate
312 and precursor layer(s) 316 may be heated from an ambient
temperature to a plateau temperature range of between about
200.degree. C. and about 600.degree. C. The temperature may be
maintained in the plateau range for a period of time ranging
between about a fraction of a second to about 60 minutes, and
subsequently reduced. The heat turns the precursor layer into a
film 322. Optionally, this may be a dense, metallic film as shown
in FIG. 2D. The heating may remove voids and create a denser film
than the precursor layer. In other embodiments, where the precursor
layer is already dense, there may be little to no
densification.
[0140] Optionally, as shown in FIG. 2D, a layer 326 containing an
additional chalcogen source, and/or an atmosphere containing a
chalcogen source, may optionally be applied to layer 322. Heat 328
may optionally be applied to layer 322 and the layer 326 and/or
atmosphere containing the chalcogen source to heat them to a
temperature sufficient to melt the chalcogen source and to react
the chalcogen source with the group IB element and group MA
elements in the precursor layer 322. The heat 328 may be applied in
a rapid thermal annealing process, e.g., as described above. The
reaction of the chalcogen source with the group IB and MA elements
forms a compound film 330 of a group IB-IIIA-chalcogenide compound.
Preferably, the group IB-IIIA-chalcogenide compound is of the form
(Ag, Au, Cu),In.sub.1-xGa.sub.xSe.sub.2(1-.alpha.)S.sub.2y, where
0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and
0.5.ltoreq.y.ltoreq.1.5. Although not limited to the following, the
compound film 330 may be thicker than the film 322 due to the
reaction with group VIA elements.
[0141] Referring now to FIGS. 2A-2E, it should be understood that
sodium may also be used with the precursor material to improve the
qualities of the resulting film. This may be particularly useful in
situation where solid Group IIIA particles are formed without using
a sodium based material and additional sodium is desired. In a
first method, as discussed in regards to FIGS. 2A and 2B, one or
more layers of a sodium containing material may be formed above
and/or below the precursor layer 316. The formation may occur by
solution coating and/or other techniques such as but not limited to
sputtering, evaporation, CBD, electroplating, sol-gel based
coating, spray coating, chemical vapor deposition (CVD), physical
vapor deposition (PVD), atomic layer deposition (ALD), and the
like.
[0142] Optionally, in a second method, sodium may also be
introduced into the stack by sodium doping the flakes and/or
particles in the precursor layer 316. As a nonlimiting example, the
flakes and/or other particles in the precursor layer 316 may be a
sodium containing material such as, but not limited to, Au--Na,
Ag--Na, Cu--Na, In--Na, Ga--Na, Cu--In--Na, Au--In--Na, Ag--In--Na,
Au--Ga--Na, Ag--Ga--Na, Cu--Ga--Na, In--Ga--Na, Na--Se, Au--Se--Na,
Ag--Se--Na, Cu--Se--Na, In--Se--Na, Ga--Se--Na, Au--In--Se--Na,
Ag--In--Se--Na, Cu--In--Se--Na, Au--Ga--Se--Na, Ag--Ga--Se--Na,
Cu--Ga--Se--Na, In--Ga--Se--Na, Ag--In--Ga--Se--Na,
Au--In--Ga--Se--Na, Cu--In--Ga--Se--Na, Na--S, Au--In--Ga--Na,
Ag--In--Ga--Na, Cu--In--Ga--Na, Au--S--Na, Ag--S--Na, Cu--S--Na,
In--S--Na, Ga--S--Na, Au--In--S--Na, Ag--In--S--Na, Cu--In--S--Na,
Au--Ga--S--Na, Ag--Ga--S--Na, Cu--Ga--S--Na, In--Ga--S--Na,
Au--In--Ga--S--Na, Ag--In--Ga--S--Na, and/or Cu--In--Ga--S--Na. In
one embodiment of the present invention, the amount of sodium in
the flakes and/or other particles may be about 1 at. % or less. In
another embodiment, the amount of sodium may be about 0.5 at. % or
less. In yet another embodiment, the amount of sodium may be about
0.1 at. % or less. It should be understood that the doped particles
and/or flakes may be made by a variety of methods including milling
feedstock material with the sodium containing material and/or
elemental sodium.
[0143] Optionally, in a third method, sodium may be incorporated
into the ink itself, regardless of the type of particle,
nanoparticle, microflake, and/or nanoflakes dispersed in the ink.
As a nonlimiting example, the ink may include flakes (Na doped or
undoped) and a sodium compound with an organic counter-ion (such as
but not limited to sodium acetate) and/or a sodium compound with an
inorganic counter-ion (such as but not limited to sodium sulfide).
It should be understood that sodium compounds added into the ink
(as a separate compound), might be present as particles (e.g.
nanoparticles), or dissolved and/or in (reverse) micelles. The
sodium may be in "aggregate" form of the sodium compound (e.g.
dispersed particles), and the "molecularly dissolved" form.
[0144] None of the three aforementioned methods are mutually
exclusive and may be applied singly or in any single or multiple
combination(s) to provide the desired amount of sodium to the stack
containing the precursor material. Additionally, sodium and/or a
sodium containing compound may also be added to the substrate (e.g.
into the molybdenum target). Also, sodium-containing layers may be
formed in between one or more precursor layers if multiple
precursor layers (using the same or different materials) are used.
It should also be understood that the source of the sodium is not
limited to those materials previously listed. As a nonlimiting
example, basically, any deprotonated alcohol where the proton is
replaced by sodium, any deprotonated organic and inorganic acid,
the sodium salt of the (deprotonated) acid,
Na.sub.xH.sub.ySe.sub.zS.sub.uTe.sub.vO.sub.w where x, y, z, u, v,
and w.gtoreq.O, Na.sub.xCu.sub.yIn.sub.zGa.sub.uO.sub.v where x, y,
z, u, and v.gtoreq.0 sodium hydroxide, sodium acetate, and the
sodium salts of the following acids: butanoic acid, hexanoic acid,
octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid,
hexadecanoic acid, 9-hexadecenoic acid, octadecanoic acid,
9-octadecenoic acid, 11-octadecenoic acid, 9,12-octadecadienoic
acid, 9,12,15-octadecatrienoic acid, and/or 6,9,12-octadecatrienoic
acid.
[0145] Optionally, as seen in FIG. 2E, it should also be understood
that sodium and/or a sodium compound may be added to the processed
chalcogenide film after the precursor layer has been densified or
otherwise processed. This embodiment of the present invention thus
modifies the film after ACIGS formation. With sodium, carrier trap
levels associated with the grain boundaries are reduced, permitting
improved electronic properties in the film. A variety of sodium
containing materials such as those listed above may be deposited as
layer 332 onto the processed film and then annealed to treat the
ACIGS film.
[0146] Additionally, the sodium material may be combined with other
elements that can provide a bandgap widening effect. Two elements
which would achieve this include gallium and sulfur. The use of one
or more of these elements, in addition to sodium, may further
improve the quality of the absorber layer. The use of a sodium
compound such as but not limited to Na.sub.2S, NaInS.sub.2, or the
like provides both Na and S to the film and could be driven in with
an anneal such as but not limited to an RTA step to provide a layer
with a bandgap different from the bandgap of the unmodified CIGS
layer or film.
[0147] Referring now to FIGS. 4A and 4B, embodiments of the flakes
108 according to the present invention will be described in further
detail. The flakes 108 may come in a variety of shapes and sizes.
In one embodiment, the flakes 108 may have a large aspect ratio, in
terms of particle thickness to particle length. FIG. 2A shows that
some flakes have thicknesses between about 0.2 to about 0.4 microns
(200 to 400 nm) and lengths between about 2 to about 5 microns
(2000 to 5000 nm). Optionally, some embodiments use smaller flakes
with the median size in the longest dimension in the range of about
400 nm to 700 nm. As a nonlimiting example, the plates are thin
(about 100 nm to 75 nm thickness or less) while their lengths may
be as large as about 5 microns (5000 nm). Some may have a length of
about 3 microns (3000 nm) or less. Other embodiments of the flakes
108 may have a length of about 1 micron (1000 nm) or less. The
aspect ratio in some embodiments of flakes may be about 10:1 or
more (ratio of the longest dimension to the shortest dimension of a
particle). Other embodiments may have an aspect ratio of about 30:1
or more. Still others may have an aspect ratio of about 50:1 or
more. An increase in aspect ratio would indicate that the longest
dimension has increased over the shortest dimension or that the
shortest dimension has decreased relative to the longest dimension.
Thus, aspect ratio herein involves the longest lateral dimension
(be it length or width) relative to the shortest dimension, which
is typically the thickness of a flake. The dimensions are measured
along edges or across a major axis to provide measurement of
dimensions such as but not limited to length, width, depth, and/or
diameter. When referring to a plurality of flakes having a defined
aspect ratio, what is meant is that all of the flakes of a
composition as a whole have an average aspect ratio as defined. It
should be understood that there may be a distribution of particle
aspect ratios around the average aspect ratio.
[0148] As seen in FIG. 4A, although the size and shape of the
flakes 108 may vary, most include at least one substantially planar
surface 120. The at least one planar surface 120 allows for greater
surface contact between adjacent flakes 108. The greater surface
contact provides a variety of benefits. The greater contact allows
for improved atomic intermixing between adjacent particles. For
flakes containing more than one element, even though there may be
atomic intermixing already in place for the particles, the close
contact in the film allows easy subsequent diffusion. Thus, if a
particle is slightly rich in one element, the increased contact
facilitates a more even distribution of elements in the resulting
dense film. Furthermore, greater interparticle interfacial area
leads to faster reaction rates. The planar shape of the particles
maximizes interparticle contact area. The interparticle contact
area allows chemical reactions (e.g. based for example upon atomic
diffusion) to be initiated, catalyzed, and/or progress relatively
rapidly and concurrently over large areas. Thus, not only does the
shape improve intermixing, the greater interfacial area and
interparticle contact area also improves reaction rates.
[0149] Referring still to FIG. 4A, the planar shape also allows for
improved packing density. As seen in FIG. 4A, the flakes 108 may be
oriented substantially parallel to the surface of substrate 102 and
stack one on top of the other to form the precursor layer 106.
Intrinsically, the geometry of the flakes allow for more intimate
contact than spherical particles or nanoparticles in the precursor
layer. In fact, it is possible that 100% of the planar surface of
the flake is in contact with another flake. Thus, the planar shape
of the flakes creates a higher packing density in the dense film as
compared to a film made from a precursor layer using an ink of
spherical nanoparticles of the same composition that is otherwise
substantially identical. In some embodiments, the planar shape of
the flakes creates a packing density of at least about 70% in the
precursor layer. In other embodiments, the flakes create a packing
density of at least about 80% in the precursor layer. In other
embodiments, the flakes create a packing density of at least about
90% in the precursor layer. In other embodiments, the flakes create
a packing density of at least about 95% in the precursor layer.
[0150] As seen in FIG. 4B, the flakes 108 may have a variety of
shapes. In some embodiments, the flakes in the ink may include
those that are of random size and/or random shape. On the contrary,
particles size is extremely important for standard spherical
nanoparticles, and those spherical nanoparticles of different size
and composition will result in dispersion with unstable atomic
composition. The planar surface 120 of the flakes allows for
particles that are more easily suspended in the carrier liquid.
Thus, even though the flakes may not be monodisperse in size,
putting the constituent metals in plate form provides one method to
have particles suspended in the carrier liquid without rapid and/or
preferential settling of any constituent element.
[0151] It should be understood that the flakes 108 of the present
invention may be formed and/or size discriminated to provide a more
controlled size and shape distribution. The size distribution of
flakes may be such that one standard deviation from a mean length
and/or width of the flakes is less than about 1000 nm. The size
distribution of flakes may be such that one standard deviation from
a mean length and/or width of the flakes is less than about 600 nm.
The size distribution of flakes may be such that one standard
deviation from a mean length and/or width of the flakes is less
than about 500 nm. The size distribution of flakes may be such that
one standard deviation from a mean length and/or width of the
flakes is less than about 400 nm. The size distribution of flakes
may be such that one standard deviation from a mean length and/or
width of the flakes is less than about 250 nm. In another
embodiment, the size distribution of flakes may be such that one
standard deviation from a mean length and/or width of the flakes is
less than about 100 nm. In another embodiment, one standard
deviation from a mean length of the flakes is less than about 50
nm.
[0152] In yet another embodiment, one standard deviation from a
mean thickness of the flakes is less than about 10 nm. In another
embodiment of the invention, one standard deviation from a mean
thickness of the flakes is less than about 5 nm. The flakes each
have a thickness less than about 250 nm. In another embodiment, the
flakes each have a thickness less than about 100 nm. In yet another
embodiment, the flakes each have a thickness less than about 20 nm.
The flakes may have a length of less than about 5 microns and a
thickness of less than about 250 nm. In another embodiment, the
flakes may have a length of less than about 2 microns and a
thickness of less than about 100 nm. In another embodiment, the
flakes have a length of less than about 1 micron and a thickness of
less than about 50 nm. In terms of their shape, the flakes may have
an aspect ratio of at least about 10 or more. In another
embodiment, the flakes have an aspect ratio of at least about 15 or
more. The flakes are of random planar shape and/or a random size
distribution. In other embodiments, the flakes are of non-random
planar shape and/or a non-random size distribution. Additionally,
FIG. 4C shows a magnified top-down view of nanoflakes 121 according
to one embodiment of the present invention
[0153] The stoichiometric ratio of elements may vary between
individual flakes so long as the overall amount in all of the
particles combined is at the desired or close to the desired
stoichiometric ratio for the precursor layer and/or resulting dense
film. According to one preferred embodiment of that process, the
overall amount of elements in the resulting film has a Cu/(In+Ga)
compositional range of about 0.7 to about 1.0 and a Ga/(In+Ga)
compositional range of about 0.05 to about 0.30. Optionally, the
Se/(In+Ga) compositional range may be about 0.00 to about 4.00 such
that a later step involving use of an additional source of Se may
or may not be required.
[0154] Referring now to FIG. 3, a flowchart showing one embodiment
of a method according to the present invention will now be
described. FIG. 3 shows that at step 350, the microflakes 108 may
be created using one of the processes described herein. Optionally,
there may be a washing step 351 to remove any undesired residue.
Once the microflakes 108 are created, step 352 shows that the ink
may be formulated with the microflakes and at least one other
component such as but not limited to a carrier liquid. Optionally,
it should be understood that some embodiments of the invention may
combine the steps 350 and 352 into one process step as indicated by
box 353 (shown in phantom) if the creation process results in a
coatable formulation. As one nonlimiting example, this may be the
case if the dispersants and/or solvents used during formation can
also be used to form a good coating. At step 354, the substrate 102
may be coated with the ink to form the precursor layer 106.
Optionally, there may be a step 355 of removing dispersant and/or
other residual of the as-coated layer 106 by methods such as but
not limited to heating, washing, or the like. Optionally, step 355
may involve a step of removing solve after ink deposition by using
a drying device such as but not limited to a drying tunnel/furnace.
Step 356 shows the precursor layer is processed to form a dense
film which may then further be processed at step 358 to form the
absorber layer. Optionally, it should be understood that some
embodiments of the invention may combine the steps 356 and 358 into
one process step if the dense film is an absorber layer and no
further processing of the film is needed. Step 360 shows that the
n-type junction may be formed over and/or in contact with the
absorber layer. Step 362 shows that a transparent electrode may be
formed over the n-type junction layer to create a stack that can
function as a solar cell.
[0155] To formulate the dispersion used in the precursor layer 106,
the flakes 108 are mixed together and with one or more chemicals
including but not limited to dispersants, surfactants, polymers,
binders, cross-linking agents, emulsifiers, anti-foaming agents,
dryers, solvents, fillers, extenders, thickening agents, film
conditioners, anti-oxidants, flow agents, leveling agents, and
corrosion inhibitors.
[0156] The inks created using the present invention may optionally
include a dispersant. Some embodiments may not include any
dispersants. Dispersants (also called wetting agents) are
surface-active substances used to prevent particles from
aggregating or flocculating, thus facilitating the suspension of
solid materials in a liquid medium and stabilizing the dispersion
thereby produced. If particle surfaces attract one another, then
flocculation occurs, often resulting in aggregation and decreasing
stability and/or homogeneity. If particle surfaces repel one
another, then stabilization occurs, where particles do not
aggregate and tend not to settle out of solution as fast.
[0157] An efficient dispersing agent can typically perform pigment
wetting, dispersing, and stabilizing. Dispersing agents are
different depending on the nature of the ink/paint. Polyphosphates,
styrene-maleinates and polyacrylates are often used for aqueous
formulations whereas fatty acid derivatives and low molecular
weight modified alkyd and polyester resins are often used for
organic formulations.
[0158] Surfactants are surface-active agents that lower the surface
tension of the solvent in which they dissolve, serving as wetting
agents, and keeping the surface tension of an (aqueous) medium low
so that an ink interacts with a substrate surface. Certain types of
surfactants are also used as dispersing agents. Surfactants
typically contain both a hydrophobic carbon chain and a hydrophilic
polar group. The polar group can be non-ionic. If the polar group
is ionic, the charge can be either positive or negative, resulting
in cationic or anionic surfactants. Zwitterionic surfactants
contain both positive and negative charges within the same
molecule; one example is N-n-Dodecyl-N,N-dimethyl betaine. Certain
surfactants are often used as dispersant agents for aqueous
solutions. Representative classes include acetylene diols, fatty
acid derivatives, phosphate esters, sodium polyacrylate salts,
polyacrylic acids, soya lecithin, trioctylphosphine (TOP), and
trioctylphosphine oxide (TOPO).
[0159] Binders and resins are often used to hold together proximate
particles in a nascent or formed dispersion. Examples of typical
binders include acrylic monomers (both as monofunctional diluents
and multifunctional reactive agents), acrylic resins (e.g. acrylic
polyol, amine synergists, epoxy acrylics, polyester acrylics,
polyether acrylics, styrene/acrylics, urethane acrylics, or vinyl
acrylics), alkyd resins (e.g. long-oil, medium-oil, short-oil, or
tall oil), adhesion promoters such as but not limited to polyvinyl
pyrrolidone (PVP), amide resins, amino resins (such as but not
limited to melamine-based or urea-based compounds),
asphalt/bitumen, butadiene acrylonitriles, cellulosic resins (such
as but not limited to cellulose acetate butyrate (CAB)), cellulose
acetate proprionate (CAP), ethyl cellulose (EC), nitrocellulose
(NC), or organic cellulose ester), chlorinated rubber, dimer fatty
acids, epoxy resin (e.g. acrylates, bisphenol A-based resins, epoxy
UV curing resins, esters, phenol and cresol (Novolacs), or
phenoxy-based compounds), ethylene co-terpolymers such as ethylene
acrylic/methacrylic Acid, E/AA, E/M/AA or ethylene vinyl acetate
(EVA), fluoropolymers, gelatin (e.g. Pluronic F-68 from BASF
Corporation of Florham Park, N.J.), glycol monomers, hydrocarbon
resins (e.g. aliphatic, aromatic, or coumarone-based such as
indene), maelic resins, modified urea, natural rubber, natural
resins and gums, rosins, modified phenolic resins, resols,
polyamide, polybutadienes (liquid hydroxyl-terminated), polyesters
(both saturated and unsaturated), polyolefins, polyurethane (PU)
isocyanates (e.g. hexamethylene diisocynate (HDI), isophorone
diisocyanate (IPDI), cycloaliphatics, diphenylmethane disiocyanate
(MDI), toluene diisocynate (TDI), or trimethylhexamethylene
diisocynate (TMDI)), polyurethane (PU) polyols (e.g. caprolactone,
dimer-based polyesters, polyester, or polyether), polyurethane (PU)
dispersions (PUDs) such those based on polyesters or polyethers,
polyurethane prepolymers (e.g. caprolactone, dimer-based
polyesters, polyesters, polyethers, and compounds based on urethane
acrylate), Polyurethane thermoplastics (TPU) such as polyester or
polyether, silicates (e.g. alkyl-silicates or water-glass based
compounds), silicones (amine functional, epoxy functional, ethoxy
functional, hydroxyl functional, methoxy functional, silanol
functional, or cinyl functional), styrenes (e.g. styrene-butadiene
emulsions, and styrene/vinyl toluene polymers and copolymers), or
vinyl compounds (e.g. polyolefins and polyolefin derivatives,
polystyrene and styrene copolymers, or polyvinyl acetate
(PVAC)).
[0160] Emulsifiers are dispersing agents that blend liquids with
other liquids by promoting the breakup of aggregating materials
into small droplets and therefore stabilize the suspension in
solution. For example, sorbitan esters are used as an emulsifier
for the preparation of water-in-oil (w/o) emulsions, for the
preparation of oil absorption bases (w/o), for the formation of w/o
type pomades, as a reabsorption agent, and as a non toxic
anti-foaming agent. Examples of emulsifiers are sorbitan esters
such as sorbitan sesquioleate (Arlacel 60), sorbitan sesquioleate
(Arlacel 83), sorbitan monolaurate (Span 20), sorbitan
monopalmitate (Span 40), sorbitan monostearate (Span 60), sorbitan
tristearate (Span 65), sorbitan mono-oleate (Span 80), and sorbitan
trioleate (Span 85) all of which are available, e.g., from Uniqema
of New Castle, Del. Other polymeric emulsifiers include
polyoxyethylene monostearate (Myrj 45), polyoxyethylene
monostearate (Myrj 49), polyoxyl 40 stearate (Myrj 52),
polyoxyethylene monolaurate (PEG 400), polyoxyethylene monooleate
(PEG 400 monoleate) and polyoxyethylene monostearate (PEG 400
monostearate), and the Tween series of surfactants including but
not limited to polyoxyethylene sorbitan monolaurate (Tween 20),
polyoxyethylene sorbitan monolaurate (Tween 21), polyoxyethylene
sorbitan monopalmitate (Tween 40), polyoxyethylene sorbitan
monostearate (Tween 60), polyoxyethylene sorbitan tristearate
(Tween 61), polyoxyethylene sorbitan mono-oleate (Tween 80),
polyoxyethylene sorbitan monooleate (Tween 81), and polyoxyethylene
sorbitan tri-oleate (Tween 85) all of which are available, e.g.,
from Uniqema of New Castle, Del. Arlacel, Myrj, and Tween are
registered trademarks of ICI Americas Inc. of Wilmington, Del.
[0161] Foam may form during the coating/printing process,
especially if the printing process takes place at high speeds.
Surfactants may adsorb on the liquid--air interface and stabilize
it, accelerating foam formation. Anti-foaming agents prevent
foaming from being initiated, while defoaming agents minimize or
eliminate previously-formed foam. Anti-foaming agents include
hydrophobic solids, fatty oils, and certain surfactants, all of
which penetrate the liquid-air interface to slow foam formation.
Anti-foaming agents also include both silicate, silicone and
silicone-free materials. Silicone-free materials include
microcrystalline wax, mineral oil, polymeric materials, and silica-
and surfactant-based materials.
[0162] Solvents can be aqueous (water-based) or non-aqueous
(organic). While environmentally friendly, water-based solutions
carry the disadvantage of a relatively higher surface tension than
organic solvents, making it more difficult to wet substrates,
especially plastic substrates. To improve substrate wetting with
polymer substrates, surfactants may be added to lower the ink
surface tension (while minimizing surfactant-stabilized foaming),
while the substrate surfaces are modified to enhance their surface
energy (e.g. by corona treatment). Typical organic solvents include
acetate, acrylates, alcohols (butyl, ethyl, isopropyl, or methyl),
aldehydes, benzene, dibromomethane, chloroform, dichloromethane,
dichloroethane, trichloroethane, cyclic compounds (e.g.
cyclopentanone or cyclohexanone), esters (e.g. butyl acetate or
ethyl acetate), ethers, glycols (such as ethylene glycol or
propylene glycol), hexane, heptane, aliphatic hydrocarbons,
aromatic hydrocarbons, ketones (e.g. acetone, methyl ethyl ketone,
or methyl isobutyl ketone), natural oils, terpenes, terpinol,
toluene.
[0163] Additional components may include fillers/extenders,
thickening agents, rheology modifiers, surface conditioners,
including adhesion promoters/bonding, anti-gelling agents,
anti-blocking agents, antistatic agents, chelating/complexing
agents, corrosion inhibitors, flame/rust inhibitors, flame and fire
retardants, humectants, heat stabilizers, light-stabilizers/UV
absorbers, lubricants, pH stabilizers, and materials for slip
control, anti-oxidants, and flow and leveling agents. It should be
understood that all components may be added singly or in
combination with other components.
Roll-to-Roll Manufacturing
[0164] A roll-to-roll manufacturing process according to the
present invention will now be described. Embodiments of the
invention using the solid group IIIA-based materials are well
suited for use with roll-to-roll manufacturing. Specifically, in a
roll-to-roll manufacturing system 400 a flexible substrate 401,
e.g., aluminum foil travels from a supply roll 402 to a take-up
roll 404. In between the supply and take-up rolls, the substrate
401 passes a number of applicators 406A, 406B, 406C, e.g. gravure
rollers and heater units 408A, 408B, 408C. It should be understood
that these heater units may be thermal heaters or be laser
annealing type heaters as described herein. Each applicator
deposits a different layer or sub-layer of a precursor layer, e.g.,
as described above. The heater units are used to anneal the
different layers and/or sub-layers to form dense films. In the
example depicted in FIG. 7, applicators 406A and 406B may apply
different sub-layers of a precursor layer. Heater units 408A and
408B may anneal each sub-layer before the next sub-layer is
deposited. Alternatively, both sub-layers may be annealed at the
same time. Applicator 406C may optionally apply an extra layer of
material containing chalcogen or alloy or elemental particles as
described above. Heater unit 408C heats the optional layer and
precursor layer as described above. Note that it is also possible
to deposit the precursor layer (or sub-layers) then deposit any
additional layer and then heat all three layers together to form
the IB-IIIA-chalcogenide compound film used for the photovoltaic
absorber layer. The roll-to-roll system may be a continuous
roll-to-roll and/or segmented roll-to-roll, and/or batch mode
processing.
Photovoltaic Device
[0165] Referring now to FIG. 8, the films fabricated as described
above using solid group IIIA-based materials may serve as an
absorber layer in a photovoltaic device, module, or solar panel. An
example of such a photovoltaic device 450 is shown in FIG. 8. The
device 450 includes a base substrate 452, an optional adhesion
layer 453, a base or back electrode 454, a p-type absorber layer
456 incorporating a film of the type described above, an n-type
semiconductor thin film 458 and a transparent electrode 460. By way
of example, the base substrate 452 may be made of a metal foil, a
polymer such as polyimides (PI), polyamides, polyetheretherketone
(PEEK), Polyethersulfone (PES), polyetherimide (PEI), polyethylene
naphtalate (PEN), Polyester (PET), related polymers, a metallized
plastic, and/or combination of the above and/or similar materials.
By way of nonlimiting example, related polymers include those with
similar structural and/or functional properties and/or material
attributes. The base electrode 454 is made of an electrically
conductive material. By way of example, the base electrode 454 may
be of a metal layer whose thickness may be selected from the range
of about 0.1 micron to about 25 microns. An optional intermediate
layer 453 may be incorporated between the electrode 454 and the
substrate 452. The transparent electrode 460 may include a
transparent conductive layer 459 and a layer of metal (e.g., Al,
Ag, Cu, or Ni) fingers 461 to reduce sheet resistance. Optionally,
the layer 453 may be a diffusion barrier layer to prevent diffusion
of material between the substrate 452 and the electrode 454. The
diffusion barrier layer 453 may be a conductive layer or it may be
an electrically nonconductive layer. As nonlimiting examples, the
layer 453 may be composed of any of a variety of materials,
including but not limited to chromium, vanadium, tungsten, and
glass, or compounds such as nitrides (including tantalum nitride,
tungsten nitride, titanium nitride, silicon nitride, zirconium
nitride, and/or hafnium nitride), oxides, carbides, and/or any
single or multiple combination of the foregoing. Although not
limited to the following, the thickness of this layer can range
from 10 nm to 50 nm. In some embodiments, the layer may be from 10
nm to 30 nm. Optionally, an interfacial layer may be located above
the electrode 454 and be comprised of a material such as including
but not limited to chromium, vanadium, tungsten, and glass, or
compounds such as nitrides (including tantalum nitride, tungsten
nitride, titanium nitride, silicon nitride, zirconium nitride,
and/or hafnium nitride), oxides, carbides, and/or any single or
multiple combination of the foregoing.
[0166] The n-type semiconductor thin film 458 serves as a junction
partner between the compound film and the transparent conducting
layer 459. By way of example, the n-type semiconductor thin film
458 (sometimes referred to as a junction partner layer) may include
inorganic materials such as cadmium sulfide (CdS), zinc sulfide
(ZnS), zinc hydroxide, zinc selenide (ZnSe), n-type organic
materials, or some combination of two or more of these or similar
materials, or organic materials such as n-type polymers and/or
small molecules. Layers of these materials may be deposited, e.g.,
by chemical bath deposition (CBD) and/or chemical surface
deposition (and/or related methods), to a thickness ranging from
about 2 nm to about 1000 nm, more preferably from about 5 nm to
about 500 nm, and most preferably from about 10 nm to about 300 nm.
This may also be configured for use in a continuous roll-to-roll
and/or segmented roll-to-roll and/or a batch mode system.
[0167] The transparent conductive layer 459 may be inorganic, e.g.,
a transparent conductive oxide (TCO) such as but not limited to
indium tin oxide (ITO), fluorinated indium tin oxide, zinc oxide
(ZnO) or aluminum doped zinc oxide, or a related material, which
can be deposited using any of a variety of means including but not
limited to sputtering, evaporation, chemical bath deposition (CBD),
electroplating, sol-gel based coating, spray coating, chemical
vapor deposition (CVD), physical vapor deposition (PVD), atomic
layer deposition (ALD), and the like. Alternatively, the
transparent conductive layer may include a transparent conductive
polymeric layer, e.g. a transparent layer of doped PEDOT
(Poly-3,4-Ethylenedioxythiophene), carbon nanotubes or related
structures, or other transparent organic materials, either singly
or in combination, which can be deposited using spin, dip, or spray
coating, and the like or using any of various vapor deposition
techniques. Optionally, it should be understood that intrinsic
(non-conductive) i-ZnO or other intrinsic transparent oxide may be
used between CdS and Al-doped ZnO. Combinations of inorganic and
organic materials can also be used to form a hybrid transparent
conductive layer. Thus, the layer 459 may optionally be an organic
(polymeric or a mixed polymeric-molecular) or a hybrid
(organic-inorganic) material. Examples of such a transparent
conductive layer are described e.g., in commonly-assigned US Patent
Application Publication Number 20040187317, which is incorporated
herein by reference.
[0168] Those of skill in the art will be able to devise variations
on the above embodiments that are within the scope of these
teachings. For example, it is noted that in embodiments of the
present invention, portions of the IB-IIIA precursor layers (or
certain sub-layers of the precursor layers or other layers in the
stack) may be deposited using techniques other than particle-based
inks. For example precursor layers or constituent sub-layers may be
deposited using any of a variety of alternative deposition
techniques including but not limited to solution-deposition of
spherical nanopowder-based inks, vapor deposition techniques such
as ALD, evaporation, sputtering, CVD, PVD, electroplating and the
like.
[0169] Referring now to FIG. 5A, it should also be understood that
the embodiments of the present invention may also be used on a
rigid substrate 600. By way of nonlimiting example, the rigid
substrate 600 may be glass, soda-lime glass, steel, stainless
steel, aluminum, polymer, ceramic, coated polymer, or other rigid
material suitable for use as a solar cell or solar module
substrate. A high speed pick-and-place robot 602 may be used to
move rigid substrates 600 onto a processing area from a stack or
other storage area. In FIG. 5A, the substrates 600 are placed on a
conveyor belt which then moves them through the various processing
chambers. Optionally, the substrates 600 may have already undergone
some processing by the time and may already include a precursor
layer on the substrate 600. Other embodiments of the invention may
form the precursor layer as the substrate 600 passes through the
chamber 606. Any of the foregoing may be adapted for use with a
laser annealing system that selectively processes target layers
over substrates. This may occur in one or more of the chambers
through which the substrate 600 passes.
[0170] FIG. 5B shows another embodiment of the present system where
a pick-and-place robot 610 is used to position a plurality of rigid
substrates on a carrier device 612 which may then be moved to a
processing area as indicated by arrow 614. This allows for multiple
substrates 600 to be loaded before they are all moved together to
undergo processing. Source 662 may provide a source of processing
gas to provide a suitable atmosphere to create the desired
semiconductor film. In one embodiment, chalcogen vapor may be
provided by using a partially or fully enclosed chamber with a
chalcogen source 662 therein or coupled to the chamber. Any of the
foregoing may be adapted for use with a laser annealing system that
selectively processes target layers over substrates.
Chalcogen Vapor Environment
[0171] Yet another embodiment of the present invention will now be
described. In this embodiment for use with a metal-ion based
precursor material, it should be understood that a chalcogen vapor
may be used to provide a chalcogen atmosphere to process a film
into the desired absorber layer. Optionally, in one embodiment, an
overpressure from chalcogen vapor is used to provide a chalcogen
atmosphere. FIG. 7A shows a chamber 1050 with a substrate 1052
having a layer 1054 and a precursor layer 1056. Extra sources 1058
of chalcogen may be included in the chamber and are brought to a
temperature to generate chalcogen vapor as indicated by lines 1060.
In one embodiment of the present invention, the chalcogen vapor is
provided to have a partial pressure of the chalcogen present in the
atmosphere greater than or equal to the vapor pressure of chalcogen
that would be required to maintain a partial chalcogen pressure at
the processing temperature and processing pressure to minimize loss
of chalcogen from the precursor layer, and if desired, provide the
precursor layer with additional chalcogen. The partial pressure is
determined in part on the temperature that the chamber 1050 or the
precursor layer 1056 is at. It should also be understood that the
chalcogen vapor is used in the chamber 1050 at a non-vacuum
pressure. In one embodiment, the pressure in the chamber is at
about atmospheric pressure. Per the ideal gas law PV=nRT, it should
be understood that the temperature influences the vapor pressure.
In one embodiment, this chalcogen vapor may be provided by using a
partially or fully enclosed chamber with a chalcogen source 1062
therein or coupled to the chamber. In another embodiment using a
more open chamber, the chalcogen overpressure may be provided by
supplying a source producing a chalcogen vapor. The chalcogen vapor
may serve to help keep the chalcogen in the film. Thus, the
chalcogen vapor may or may not be used to provide excess chalcogen.
It may serve more to keep the chalcogen present in the film than to
provide more chalcogen into the film.
[0172] In yet another embodiment, it is shown that the present
invention may be adopted for use with a roll-to-roll system where
the substrate 1070 carrying the precursor layer may be flexible and
configured as rolls 1072 and 1074. The chamber 1076 may be at
vacuum or non-vacuum pressures. The chamber 1076 may be designed to
incorporate a differential valve design to minimize the loss of
chalcogen vapor at the chamber entry and chamber exit points of the
roll-to-roll substrate 1070.
[0173] In a still further embodiment of the present invention, the
system uses a chamber 1090 of sufficient size to hold the entire
substrate, including any rolls 1072 or 1074 associated with using a
roll-to-roll configuration.
Extra Source of Chalcogen
[0174] It should be understood that the present invention using
metal ion precursors or hydroxides may also use an extra chalcogen
source in a manner similar to that described in copending, U.S.
patent application Ser. No. 11/290,633 (Attorney Docket No.
NSL-045), wherein the precursor material contains the previous
materials and 1) chalcogenides such as, but not limited to, copper
selenide, and/or indium selenide and/or gallium selenide and/or 2)
a source of extra chalcogen such as, but not limited to, Se or S
nanoparticles less than about 200 nanometers in size. In one
nonlimiting example, the chalcogenide and/or the extra chalcogen
may be in the form of microflakes and/or nanoflakes while the extra
source of chalcogen may be flakes and/or non-flakes. The
chalcogenide microflakes may be one or more binary alloy
chalcogenides such as, but not limited to, group IB-binary
chalcogenide nanoparticles (e.g. group IB non-oxide chalcogenides,
such as Cu--Se, Cu--S or Cu--Te) and/or group IIIA-chalcogenide
nanoparticles (e.g., group IIIA non-oxide chalcogenides, such as
Ga(Se, S, Te), In(Se, S, Te) and Al(Se, S, Te). In other
embodiments, the microflakes may be non-chalcogenides such as but
not limited to group IB and/or IIIA materials like Cu--In, Cu--Ga,
and/or In--Ga. If the chalcogen melts at a relatively low
temperature (e.g., 220.degree. C. for Se, 120.degree. C. for S) the
chalcogen is already in a liquid state and makes good contact with
the microflakes. If the microflakes and chalcogen are then heated
sufficiently (e.g., at about 375.degree. C.), the chalcogen reacts
with the chalcogenides to form the desired IB-IIIA-chalcogenide
material.
[0175] Although not limited to the following, the chalcogenide
particles may be obtained starting from a binary chalcogenide
feedstock material, e.g., micron size particles or larger. Examples
of chalcogenide materials available commercially are listed below
in Table I.
TABLE-US-00002 TABLE I Chemical Formula Typical % Purity Aluminum
selenide Al2Se3 99.5 Aluminum sulfide Al2S3 98 Aluminum sulfide
Al2S3 99.9 Aluminum telluride Al2Te3 99.5 Copper selenide Cu--Se
99.5 Copper selenide Cu2Se 99.5 Gallium selenide Ga2Se3 99.999
Copper sulfide Cu2S(may be Cu1.8-2S) 99.5 Copper sulfide CuS 99.5
Copper sulfide CuS 99.99 Copper telluride CuTe(generally Cu1.4Te)
99.5 Copper telluride Cu2Te 99.5 Gallium sulfide Ga2S3 99.95
Gallium sulfide GaS 99.95 Gallium telluride GaTe 99.999 Gallium
telluride Ga2Te3 99.999 Indium selenide In2Se3 99.999 Indium
selenide In2Se3 99.99% Indium selenide In2Se3 99.9 Indium selenide
In2Se3 99.9 Indium sulfide InS 99.999 Indium sulfide In2S3 99.99
Indium telluride In2Te3 99.999 Indium telluride In2Te3 99.999
[0176] Examples of chalcogen powders and other feedstocks
commercially available are listed in Table II below.
TABLE-US-00003 TABLE II Chemical Formula Typical % Purity Selenium
metal Se 99.99 Selenium metal Se 99.6 Selenium metal Se 99.6
Selenium metal Se 99.999 Selenium metal Se 99.999 Sulfur S 99.999
Tellurium metal Te 99.95 Tellurium metal Te 99.5 Tellurium metal Te
99.5 Tellurium metal Te 99.9999 Tellurium metal Te 99.99 Tellurium
metal Te 99.999 Tellurium metal Te 99.999 Tellurium metal Te 99.95
Tellurium metal Te 99.5
Printing a Layer of the Extra Source of Chalcogen
[0177] Referring now to FIG. 1C, another embodiment of the present
invention will now be described. An extra source of chalcogen may
be provided as a discrete layer 107 containing an extra source of
chalcogen such as, but not limited to, elemental chalcogen
particles over a microflake or non-flake precursor layer. By way of
example, and without loss of generality, the chalcogen particles
may be particles of selenium, sulfur or tellurium. Heat is applied
to the precursor layer and the layer 107 containing the chalcogen
particles to heat them to a temperature sufficient to melt the
chalcogen particles and to react the chalcogen particles with the
elements in the precursor layer 106. It should be understood that
the microflakes may be made of a variety of materials include but
not limited to group IB elements, group IIIA elements, and/or group
VIA elements. The reaction of the chalcogen particles 107 with the
elements of the precursor layer 106 forms a compound film 110 of a
group IB-IIIA-chalcogenide compound. Preferably, the group
IB-IIIA-chalcogenide compound is of the form (Ag, Au,
Cu)In.sub.1-xGa.sub.xSe.sub.2(1-y)S.sub.2y, where
0.ltoreq.x.ltoreq.1 and 0.ltoreq.y.ltoreq.1. In some embodiments,
the Ag or Au is 5-15 at % of the group IB elements. It should be
understood that in some embodiments, the precursor layer 106 may be
densified prior to application of the layer 107 with the extra
source of chalcogen. In other embodiments, the precursor layer 106
is not pre-heated and the layers 106 and 107 are heated
together.
[0178] In one embodiment of the present invention, the precursor
layer 106 may be between about 4.0 to about 0.5 microns thick. The
layer 107 containing chalcogen particles may have a thickness in
the range of about 4.0 microns to about 0.5 microns. The chalcogen
particles in the layer 107 may be between about 1 nanometer and
about 25 microns in size, preferably between about 25 nanometers
and about 300 nanometers in size. It is noted that the chalcogen
particles may be initially larger than the final thickness of the
IB-IIIA-VIA compound film 110. The chalcogen particles 108 may be
mixed with solvents, carriers, dispersants etc. to prepare an ink
or a paste that is suitable for wet deposition over the precursor
layer 106 to form the layer. Alternatively, the chalcogen particles
may be prepared for deposition on a substrate through dry processes
to form the layer 107. It is also noted that the heating of the
layer 107 containing chalcogen particles may be carried out by an
RTA process, e.g., as described above.
[0179] The chalcogen particles (e.g., Se or S) may be formed in
several different ways. They may be formed by grinding, milling,
electroexplosive wire (EEW) processing, evaporation condensation
(EC), pulsed plasma processing, or combinations thereof. The
particles may be formed using at least one of the following
methods: sonification, agitation, electromagnetically mixing of a
liquid metal or liquid alloy. The particles may be formed using at
least one of the following methods: spray-pyrolysis, laser
pyrolysis, or a bottom-up technique like wet chemical
approaches.
[0180] As seen in FIG. 9A, it should also be understood that in
some embodiments, the layer 1108 of chalcogen particles may be
formed below the precursor layer 1106. This position of the layer
1108 still allows the chalcogen particles to provide a sufficient
surplus of chalcogen to the precursor layer 1106 to fully react
with the group IB and group IIIA elements in layer 1106.
Additionally, since the chalcogen released from the layer 1108 may
be rising through the layer 1106, this position of the layer 1108
below layer 1106 may be beneficial to generate greater intermixing
between elements. The thickness of the layer 1108 may be in the
range of about 4.0 microns to about 0.5 microns. In still other
embodiments, the thickness of layer 1108 may be in the range of
about 500 nm to about 50 nm. In one nonlimiting example, a separate
Se layer of about 100 nm or more might be sufficient. The coating
of chalcogen may incorporate coating with powder, Se evaporation,
or other Se deposition method such as but not limited to chemical
vapor deposition (CVD), physical vapor deposition (PVD), atomic
layer deposition (ALD), electroplating, and/or similar or related
methods using singly or in combination. Other types of material
deposition technology may be used to get Se layers thinner than 0.5
microns or thinner than 1.0 micron. It should also be understood
that in some embodiments, the extra source of chalcogen is not
limited to only elemental chalcogen, but in some embodiments, may
be an alloy and/or solution of one or more chalcogens.
[0181] Optionally, it should be understood that the extra source of
chalcogen may be mixed with and/or deposited within the precursor
layer, instead of as a discrete layer. In one embodiment of the
present invention, oxygen-free particles or substantially
oxygen-free particles of chalcogen could be used. If the chalcogen
is used with microflakes and/or plate shaped precursor materials,
densification might not end up an issue due to the higher density
achieved by using planar particles, so there is no reason to
exclude printing Se and/or other source of chalcogen within the
precursor layer as opposed to a discrete layer. This may involve
not having to heat the precursor layer to the previous processing
temperatures. In some embodiments, this may involve forming the
film without heating above 400.degree. C. In some embodiments, this
may involve not having to heat above about 300.degree. C.
[0182] In still other embodiments of the present invention,
multiple layers of material may be printed and reacted with
chalcogen before deposition of the next layer. One nonlimiting
example would be to deposit a Cu--In--Ga layer, anneal it, then
deposit an Se layer then treat that with RTA, follow that up by
depositing another precursor layer rich in Ga, followed by another
deposition of Se, and finished by a second RTA treatment. More
generically, this may include forming a precursor layer (either
heat or not) then coating a layer of the extra source of chalcogen
(then heat or not) then form another layer of more precursor (heat
or not) and then for another layer of the extra source of chalcogen
(then heat or not) and repeat as many times as desired to grade the
composition or nucleating desired crystal sizes. In one nonlimiting
example, this may be used to grade the gallium concentration. In
another embodiment, this may be used to grade the copper
concentration. In yet another embodiment, this may be used to grade
the indium concentration. In a still further embodiment, this may
be used to grade the selenium concentration. In yet another
embodiment this may be used to grade the selenium concentration.
Another reason would be to first grow copper rich films to get big
crystals and then to start adding copper-poor layers to get the
stoichiometry back. Of course this embodiment can combined to allow
the chalcogen to be deposited in the precursor layer for any of the
steps involved.
[0183] FIG. 9B shows core-shell microflakes in which the core is a
microflake 1107 and the shell 1120 is a chalcogen and/or
chalcogenide coating. Up to 10% to 8% by weight of the particle may
be oxygen. Up to 8% to 6% by weight of the particle may be oxygen.
In some embodiments, the shell is an oxide shell that is the same
or different from the material in the core. This oxygen may be
concentrated in the shell. Optionally, it may be dispersed through
the particle. Optionally, it may be in both the shell and through
out the particle. As a nonlimiting example, the core may be a mix
of elemental particles of groups IB (e.g., Cu) and/or MA (e.g., Ga
and In), which may be obtained by size reducing of feedstock to a
desired size. Examples of elemental feedstock materials available
are listed in Table III below. The core may also be a chalcogenide
core or other material as described herein.
TABLE-US-00004 TABLE III Chemical Formula Typical % Purity Copper
metal Cu 99.99 Copper metal Cu 99 Copper metal Cu 99.5 Copper metal
Cu 99.5 Copper metal Cu 99 Copper metal Cu 99.999 Copper metal Cu
99.999 Copper metal Cu 99.9 Copper metal Cu 99.5 Copper metal Cu
99.9 (O.sub.2 typ. 2-10%) Copper metal Cu 99.99 Copper metal Cu
99.997 Copper metal Cu 99.99 Gallium metal Ga 99.999999 Gallium
metal Ga 99.99999 Gallium metal Ga 99.99 Gallium metal Ga 99.9999
Gallium metal Ga 99.999 Indium metal In 99.9999 Indium metal In
99.999 Indium metal In 99.999 Indium metal In 99.99 Indium metal In
99.999 Indium metal In 99.99 Indium metal In 99.99
Absorbers with Non-Toxic Materials
[0184] It should be understood that in the foregoing, though
described in the context of group IB-IIIA-VIA absorber layers, the
inventions therein are also applicable to absorbers formed from
other nanoparticles. By way of example, there are a few classes of
compounds that are composed of abundant non-toxic materials and
have great promise for cost effective thin film solar cells such as
IB-IIB-IVA-VIA absorbers. For any of the embodiments herein, it is
also possible to have two or more elements of IB elements in the
chalcogenide particle and/or the resulting intermediate film and/or
final absorber layer.
[0185] Examples of such classes are metal-sulfides (e.g. FeS2,
NiS2, SnS, ZnS, etc.), metal-selenides (e.g. FeSe2, Ni3Se2, SnSe2,
etc.), metal-oxides (e.g. Fe2O3, Co--Fe--Al-oxides, etc.), and
I2-II-IV-VI4 compounds (e.g. Cu2ZnGeSe4, Cu2ZnSnS4, Cu2ZnSnSe4,
Cu2ZnSiSe4, etc.). Optionally, (Ag, Au, Cu)--Zn--Sn--S--Se may be
the absorber layer. Any of the group VIA vapor techniques may be
used. This may be used for selenization or sulfidation of a group
I2-II-IV precursor layer. The vapor may be at atmospheric, below
atmospheric, or above atmospheric pressure. The following
embodiments may be used on wide web metal or other flexible
substrates. Some embodiments may be 1 meter wide or more.
Optionally, another embodiment may be up to 2 meter wide or more.
Optionally, another embodiment use be up to 3 meters wide or more.
These wide webs are applicable to any of the embodiments
herein.
[0186] To create cost-effective thin film solar cells, it is
desirable that the component materials be low cost (abundant and
non-toxic), but the processing techniques used to deposit the films
need to be low cost as well. The aforementioned groups of materials
can be deposited by various high-cost and energy intensive
techniques (such as sputtering, CVD, and vacuum evaporation),
however, these particular materials classes also lend themselves to
inexpensive solution based deposition techniques (such as
densification of a nanoparticulate precursor ink,
electrodeposition, chemical bath deposition, and spray pyrolysis).
These solution based processes could utilize many types of
precursors (elemental, salts, oxides, etc.), and can be used either
to produce semiconductor crystals directly (in one step), or to
form metallic precursor layers which are subsequently crystallized
via reactive post-treatments (using sulfur, selenium, oxygen,
etc.).
[0187] For a thin film solar cell to cost-effectively generate
electrical power, not only does it need to be inexpensive to
fabricate, but have reasonably high power conversion efficiency as
well. The materials comprising high efficiency thin film solar
cells exhibit several key properties; the bandgap of the absorber
must be matched to the solar spectrum and the bandgap of a window
(or heterojunction partner) needs to be wider, the absorber must
strongly absorb sunlight, and all materials must have good
electrical properties (such as moderate doping densities, high
free-carrier mobilities and long free-carrier lifetimes). The
aforementioned groups of materials contain members exhibiting all
of these materials properties, and, since they are also composed of
inexpensive elements and are suitable for low-cost (solution based)
processing, these classes of materials are excellent choices for
cost-effective thin film solar cells. The following are fully
incorporated herein by reference for all purposes. 1.
http://www.webelements.com 2. Bjorn A. Andersson, "Materials
availability for large-scale thin-film photovoltaics", Progress in
Photovoltaics: Research and Applications, 8(1), p. 61-76
(2000).
[0188] A variety of paths may be used to form the desired
IB-IIB-IVA-VIA absorber material. Any of the following particles in
the examples below may use the nanoflake, microflake, or other
shaped particle as described herein. Optionally, they may be
deposited by electrodeposition, electrobath, sputtering,
evaporation, chemical bath deposition (CBD), electroplating,
sol-gel based coating, spray coating, chemical vapor deposition
(CVD), physical vapor deposition (PVD), atomic layer deposition
(ALD), spray pyrolysis, liquid melt, and/or the like.
[0189] Note that the method may be optimized by using, prior to,
during, or after the solution deposition and/or densification of
one or more of the precursor layers, any combination of (1) any
chalcogen source that can be solution-deposited, e.g. a Se or S
nanopowder mixed into the precursor layers or deposited as a
separate layer, (2) chalcogen (e.g., Se or S) evaporation, (3) an
H.sub.2Se (H2S) atmosphere, (4) a chalcogen (e.g., Se or S)
atmosphere, (5), an organo-selenium containing atmosphere, e.g.
diethylselenide (6) an H.sub.2 atmosphere, (7) another reducing
atmosphere, e.g. CO, (8) a wet chemical reduction step, (9) use of
a plasma, and a (10) heat treatment.
[0190] Optionally, any combination of number of deposition and
heating steps, RTP steps, heating in reducing atmosphere, any
combination of different techniques to deposit materials
(sequentially or simultaneously), any size and shape of particles,
any wet deposition technique (various coating, spraying, and
printing techniques), any method to remove dispersant, any type of
substrate (rigid and/or flexible), or any interconnect scheme
(monolithic integration, MWT, etc.) may be used with the present
absorber layer.
[0191] 1. Quaternary IB-IIB-IVA-VIA Nanoparticles
[0192] According to one set of embodiments of the present
invention, quaternary nanoparticles (e.g., quantum dots or quantum
rods) for making a IB-IIB-IVA-VIA ink may be fabricated by several
different approaches. Some embodiments may use non-oxide versions
of the above nanoparticles.
[0193] 2. Ternary IB-IIB-IVA Nanoparticles
[0194] According to one set of embodiments of the present
invention, ternary nanoparticles (e.g., quantum dots or quantum
rods) for making a IB-IIB-IVA ink may be fabricated by several
different approaches. Optionally, IB-IIB-VIA, IB-IVA-VIA, or
IIB-IVA-VIA may be manufactured. Some embodiments may use non-oxide
versions of the above nanoparticles.
[0195] a). Preparation From Single-Source Precursors
[0196] b). Preparation using Spray Co-Precipitation
[0197] c) Preparation using a Volatile Capping Agent
[0198] 3. Production of Binary Nanoparticles
[0199] According to one set of embodiments of the present
invention, binary nanoparticles (e.g., quantum dots or quantum
rods) for making a IB-IIB ink may be fabricated by several
different approaches. Optionally, IIB-VIA, IB-IVA, IVA-VIA, or
IB-VIA may be manufactured. Some embodiments may use non-oxide
versions of the above nanoparticles.
[0200] a) Nanoparticles Prepared in Volatile Solvent
[0201] b). Sonochemical Synthesis of Binary Quantum
Nanoparticles
[0202] 4. Intermetallics of the Binary Nanoparticles
[0203] According to one set of embodiments of the present
invention, binary nanoparticles (e.g., quantum dots or quantum
rods) for making a IB-IIB ink may be fabricated by several
different approaches. Optionally, IB-VIA, IB-IVA, IVA-VIA, or
IB-VIA may be manufactured. These binary nanoparticles are selected
from the intermetallic regions of their phase diagrams.
[0204] Of course, other embodiments may use elemental metal
nanoparticles of the foregoing with Se(S) Vapor or Powders. Some
embodiments may use metal Halides Dissolved in Chelating Agents
such as that described in U.S. application Ser. No. 10/782,017
fully incorporated herein by reference for all purposes. Others may
use metal salts as described in manners similar to that in U.S.
application Ser. No. 10/782,017. Stoichiometric balance (50% of
each, within a certain range) may also be used for particles of
group IIB-IVA alloy, or IIB elemental, or IVA elemental particles.
These may be selected to approximately replace a group IIIA
material in a group IB-IIB-IVA-VIA absorber layer.
[0205] It should also be understood that the diffusion barrier
layers as described in U.S. Patent Application 60/909,357 filed
Mar. 30, 2007, fully incorporated herein for all purposes, may also
be included herein to minimize corruption of the precursor layer
during processing at elevated temperatures.
[0206] It should also be understood that the group VIA tools
described in U.S. Application 61/012,020 may be used for
sulfidation, selenization, or other group VIA processing for any
group IB-IIB-IVA-VIA or IB-IIB-IVA precursor. Some may require
additional group VIA material or chalcogen rich chalcogenide
material as described in U.S. application Ser. Nos. 11/395,426,
11/290,633, 11/361,522, and/or 11/361,515, each fully incorporated
herein by reference for all purposes. The group IB-IIB-IVA
precursor would replace any IB-IIIA precursors in those
applications. Optionally, IB-IIB-IVA-VIA would replace IB-IIIA-VIA
precursors in those applications. Optionally, IB-IIB or IB-IVA
would replace any IB-IIIA precursor. Some may use IIB-IVA to
replace any IIIA-IIIA precursor used in the incorporated
applications.
[0207] It should be understood that other absorber layer materials
may also be used. For any of the embodiments herein, it is also
possible to have two or more elements of IB elements in the
chalcogenide particle and/or the resulting intermediate film and/or
final absorber layer.
TABLE-US-00005 Indium Replacement Materials (Absorbers): Cu2ZnSiTe4
Eg = 1.47 eV Cu2ZnGeSe4 Eg = 1.63 eV Cu2ZnSnS4 Eg = 1.39 eV
CdS/CZTS 5.5% PCE or higher Cu2ZnSnSe4 Eg = 1.44 eV Cu2ZnSnS4 Eg =
??? Cu2CdGeSe4 Eg = 1.20 eV Cu2CdSnS4 Eg = 1.37 eV Cu2CdSnSe4 Eg =
0.96 eV
[0208] Many binary, ternary and quaternary Metal Sulfides and
Selenides
[0209] Some binary, ternary and quaternary Metal Oxides (tend to be
wider bandgap materials)
[0210] Optionally, the material used for a window layer or junction
partner material may also be replaced with one or more of the
following:
TABLE-US-00006 Cadmium Replacement Materials (Windows/Junction
Partners): ZnS Eg = 3.6 eV ZnS/CIGS 18.6% PCE In(OH)3 Eg =
In(OH)3/CIGS 14% In2S3 Eg = In2S3/CIGS 16.4% ZnSe Eg = ZnSe/CIGS
15.7% ZnInSe Eg = ZnInSe 15.3% InSe Eg = InSe/CIGS 13.0% ZnMgO Eg =
ZnMgO/CIGS 16.2% ZnO Eg = ZnO/CIGS 15.0% SnO2 Eg = SnO2 12.2%
Cu2ZnSiS4 Eg = 3.25 eV Cu2ZnSiSe4 Eg = 2.33 eV Cu2ZnGeS4 Eg = 2.10
eV
[0211] Many binary, ternary and quaternary Metal Sulfides,
Selenides and Oxides
[0212] In one embodiment, metal ratios necessary for a CZTS solar
cell precursor foil may be (Cu,Ag,Au: 50 at.-%, Zn: 25 at.-%, Sn:
25 at.-%)
[0213] Optionally, a further object of the invention is to provide
a thin film of CZTS ((Au,Ag,Cu).sub.2ZnSnS.sub.4) and related
compounds like
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zCh1.sub.aCh2.sub.b.
[0214] A still further object of the present invention is to
provide precursors of these chalcogenides, i.e., more specifically,
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.z in foil form. In this manner, the
desired stoichiometry is fixed in the bulk material.
[0215] Thus, more specifically, an object of the present invention
is also to provide a method of forming
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.z,
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a (ACZTS),
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zSe.sub.b (ACZTSe) or
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.aSe.sub.b (ACZTSSe) layers
with well-defined total bulk stoichiometries, wherein x ranges from
1.5 to 2.5, y ranges from 0.9 to 1.5, z ranges from 0.5 to 1.1, a
ranges from 0 to 4.2, preferably from 0.1 to 4.2, and b ranges from
0 to 4.2, preferably from 0.1 to 4.2, and which method is easy to
apply and suitable for large scale production of thin film solar
cells.
[0216] As used herein after and in the claims, the term
"copper-zinc-tin alloy" denotes an alloy comprising the indicated
elements and optionally additionally at least one chalcogen. Such
alloy comprising at least one chalcogen may, if chalcogen is sulfur
which is comprised in the alloy ((Au,Ag,Cu).sub.2ZnSnS.sub.4), be
present in the stannite type structure (kesterite).
[0217] A copper-zinc-tin alloy substrate may be provided. More
specifically a (Au,Ag,Cu).sub.xZn.sub.ySn.sub.z alloy
(1.5<x<2.5; 0.9<y<1.5; 0.5<z<1.1), is used for
depositing this alloy into a suitable electrically conductive
substrate.
[0218] Optionally, the copper-zinc-tin alloy is a
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zCh1.sub.aCh2.sub.b alloy, with Ch1
being a first chalcogen, Ch2 being a second chalcogen and wherein x
is from 1.5 to 2.5, y is from 0.9 to 1.5, z is from 0.5 to 1.1, a
is from 0 to 4.2 and b is from 0 to 4.2.
[0219] Optionally, the copper, zinc and tin species are comprised
in the composition at a molar ratio of (Au,Ag,Cu):
zinc:tin=1:0.1-10:0.1-4, more specifically 2-8:0.4-1.
[0220] In a further preferred embodiment of the present invention,
the copper-zinc-tin alloy comprises Ch1, wherein Ch1 may be sulfur.
Thus, when a >0, Ch1 is S and at least one chalcogen plating
species is a sulfur plating species (sulfur source:
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a or
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b: 1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2;
0<b<4.2).
[0221] In a further preferred embodiment of the present invention,
when b>0, Ch2 is Se and at least one chalcogen plating species
is a selenium plating species (selenium source:
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zSe.sub.b or
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b: 1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0<a <4.2; 0.1<b<4.2).
Said selenium species is selected from the group comprising
selenates, selenosulfites, diselenides and polyselenides. By using
this selenium source, selenium is incorporated into the deposited
alloy layer to form (Au,Ag,Cu).sub.xZn.sub.ySn.sub.zSe.sub.b, with
x being from 1.5 to 2.5, y being from 0.9 to 1.5, z being from 0.5
to 1.1 and b being from 0.1 to 4.2.
[0222] In a further preferred embodiment of the present invention,
the copper-zinc-tin alloy comprises both Ch1 and Ch2, wherein Ch1
may be sulfur and Ch2 may be selenium. In such case the
copper-zinc-tin alloy may be
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b with x, y, z being
as before and a and b being each from 0 to 4.2.
[0223] More specifically, the bath additionally may contain a
mixture of the sulfur and selenium sources to deposit layers which
contain sulfur as well as selenium in order to form layers of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2;
0.1<b<4.2).
[0224] By using the sulfur and the selenium sources, sulfur and
selenium is are incorporated into the foil to form
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b, with x being from
1.5 to 2.5, y being from 0.9 to 1.5, z being from 0.5 to 1.1, a
being from 0.1 to 4.2 and b being from 0.1 to 4.2. The depth of the
conversion of the foil may be less than 5 microns, optionally 2
microns or less on any surface of the precursor foil.
[0225] Further, the method according to the second aspect of the
invention comprises depositing a copper-zinc-tin alloy, said alloy
optionally additionally containing at least one chalcogen, more
specifically a (Au,Ag,Cu).sub.xZn.sub.ySn.sub.z alloy
(1.5<x<2.5; 0.9<y<1.5; 0.5<z<1.1), a
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a alloy (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2), a
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2Se.sub.b alloy (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<b<4.2) or a
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b alloy
(1.5<x<2.5; 0.9<y<1.5; 0.5<z<1.1; 0.1<a
<4.2; 0.1<b<4.2).
[0226] Optionally, the foil may be treated to deposit a
(Au,Ag,Cu)-zinc-tin alloy, the substrate is optionally contacted
with an electrolytic bath at a temperature of from about 15.degree.
C. to about 80.degree. C. to form such alloy.
[0227] Optionally, the method according to the second aspect of the
present invention further comprises sulfurizing the copper-zinc-tin
alloy of the foil by contacting same with a sulfur plating species.
More preferably, said sulfur plating species is selected from the
group comprising elemental sulfur and a reducing atmosphere
containing a sulfur compound.
[0228] Optionally, the method according to the second aspect of the
present invention further comprises depositing a selenium monolayer
onto the alloy.
[0229] Optionally, the method comprises selenizing the
copper-zinc-tin alloy by contacting same with a reducing atmosphere
containing a selenium compound.
[0230] Deposition of the (Au,Ag,Cu), zinc and tin monolayers in the
method according to the fourth aspect of the present invention
which results in forming a sandwich layer is performed preferably
electrolytically. In this embodiment of the present invention the
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.z sandwich layer or the
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zSe.sub.b sandwich layer is obtained
by stepwise wet-chemical deposition of thin monolayers of copper,
tin, zinc and, optionally, selenium on a suitable substrate with a
metallic back contact.
[0231] Optionally, depositing at least one chalcogen to at least
one of said monolayers comprises sulfurizing either at least one of
said monolayers with a sulfur plating species or sulfurizing said
(Au,Ag,Cu)-zinc-tin sandwich layer by contacting same with a sulfur
plating species.
[0232] More specifically, for preparing a layer of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 3.8<a <4.2) the following
method steps may be performed: [0233] i. Preparing layers of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.z or of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a by using a method in which
a layer of (Au,Ag,Cu).sub.xZn.sub.ySn.sub.z or of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.a (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2) is prepared by
contacting a substrate with a suitable electrolytic bath at a
temperature of from 15.degree. C. to 80.degree. C. and at a pH of
from 8 to 13, or preparing sandwich layers consisting of copper,
tin and zinc monolayers and having an overall composition of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.z (1.5<x<2.5; 0.9<y<1.5;
0.5<z<1.1) by sequentially depositing stacked copper, tin and
zinc onto a substrate having a layer thickness suitable to achieve
the desired stoichiometry by contacting the substrate with an
electrolytic bath for the deposition of copper, with an
electrolytic bath for the deposition of tin and with an
electrolytic bath for the deposition of zinc in any order and any
number of stackings; and thereafter [0234] ii. Sulfurizing the
layers by contacting them with a sulfur-containing compound.
[0235] Optionally, the sulfur species is selected from the group
comprising elemental sulfur and a reducing atmosphere containing a
sulfur compound.
[0236] Optionally, sulfurization may be performed by contacting the
layers herein above with a reducing sulfur atmosphere like H.sub.2S
a temperatures higher than room temperature.
[0237] Optionally, sulfurization may be performed by contacting the
layers herein above with elemental sulfur either at room
temperature or at elevated temperatures higher than room
temperature.
[0238] Optionally, depositing at least one chalcogen to at least
one of said monolayers comprises depositing a selenium monolayer
onto the sandwich layer.
[0239] More specifically, for preparing a layer of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.aSe.sub.b (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2;
0.1<b<4.2) the following method steps may be performed:
[0240] i. Preparing layers of (Au,Ag,Cu).sub.xZn.sub.ySn.sub.z,
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a,
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zSe.sub.b or
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.aSe.sub.b (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2;
0.1<b<4.2) by contacting a substrate with a suitable
electrolytic bath at a temperature of from 15.degree. C. to
80.degree. C. and at a pH of from 8 to 13, more preferably from 10
to 12, or preparing sandwich layers consisting of copper, tin and
zinc and having an overall composition of
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2 (1.5<x<2.5; 0.9<y<1.5;
0.5<z<1.1) by sequentially depositing stacked metal layers
onto a substrate having a layer thickness suitable to achieve the
desired stoichiometry by contacting the substrate with an
electrolytic bath for the deposition of copper, with an
electrolytic bath for the deposition of tin and with an
electrolytic bath for the deposition of zinc in any order and any
number of stackings; and thereafter [0241] ii. Depositing a
selenium layer and sulfurizing the layers by contacting them with a
sulfur-containing compound.
[0242] Optionally in this latter method depositing the selenium
layer comprises depositing said layer using a (wet-chemical
reaction) electrochemical reaction by either electroless or
electrochemical deposition.
[0243] Optionally, depositing at least one chalcogen to at least
one of said monolayers comprises selenizing the sandwich layer by
contacting same with a reducing atmosphere containing a selenium
compound.
[0244] According to yet another aspect of the present invention, a
thin film solar cell is provided which comprises a substrate film,
optionally a barrier layer such as but not limited to an
electrically conductive barrier layer is deposited on the
substrate, which serves as a diffusion barrier to prevent any
constituent of the substrate to diffuse there through, an
electrically conductive back contact layer, a p-type absorber
layer, comprised of a copper-zinc-tin alloy further comprising at
least one chalcogen and having chemical formula
(Au,Ag,Cu).sub.xZn.sub.xSn.sub.2S.sub.3 with x being from 1.5 to
2.5, y being from 0.9 to 1.5, z being from 0.5 to 1.1 and a being
from 3.8 to 4.2, or having chemical formula
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.aSe.sub.b with x being from
1.5 to 2.5, y being from 0.9 to 1.5, z being from 0.5 to 1.1, a
being from 0.1 to 4.2 and b being from 0.1 to 4.2, at least one
n-type buffer layer and at least one window layer. Further, a grid
layer may be provided for electrical contact. Of course, some
embodiments may use an electrically insulating barrier layer.
[0245] More specifically, the thin film solar cell may comprise: a
substrate layer which is either electrically conductive or
non-conductive and which is furthermore either flexible or rigid;
optionally, a barrier layer which serves either as an electrical
isolator or as a diffusion barrier to prevent diffusion of any
constituent of the substrate material into the absorber layer
deposited thereon; an electrically conductive back contact layer
which is preferably made from molybdenum; the p-type absorber layer
made from (Au,Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.a (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 3.8<a <4.2), or
(Au,Ag,Cu).sub.xZn.sub.ySn.sub.2S.sub.aSe.sub.b (1.5<x<2.5;
0.9<y<1.5; 0.5<z<1.1; 0.1<a <4.2;
0.1<b<4.2) obtained by any method described herein above to
prepare such alloy;--at least one n-type buffer layer; one or more
window layers.
[0246] In one embodiment, it should be understood that the
precursor layer is designed to be IIB rich. This is particularly
true if the IIB material is Zn. The desirable atomic ratio of IIB
to IVA is at least 55:45 or optionally as high as 60:40. This IIB
rich ratio in the precursor is particularly desirable due to the
nature of the processing that occurs. Using particles such as but
not limited to elemental particles, the ratio of IIB material to
others is controllable. Optionally, some embodiments may have the
ratio locked into alloy particles of IIB-IVA, IB-IIB, IB-IVA or
IB-IIB-IVA. This IIB rich composition may also be obtained by
printing an additional layer of IIB over any existing IB IIB IVA
precursor.
[0247] Optionally, some embodiment may desire to lock in the IIB
material be using core-shell particles with JIB in the core and a
IB, IVA, or VIA shell. Optionally, some embodiments may use a IB,
IVA, or VIA layer deposited over a precursor layer of IB IIB
IVA.
[0248] In order to prepare the layers in accordance with the
present invention the substrate surface to receive such layers will
normally be subjected to a process of pre-cleaning same prior to
metallization. The substrates may be treated before plating with
wet-chemical processes developed by the applicant or with any other
cleansing chemicals, in order to remove any grease, dirt dust or
oxide from the surface. One pre-cleaning process is described in
Table 1:
[0249] Cleaning Substrate
[0250] Uniclean.RTM. 399 is a mild alkaline, slightly foaming
cleaner, which contains carbonate, silicates, phosphates, tensides
and the biodegradable chelating agent gluconate. This bath is
designed to remove mineral oils, polish and grind residues and
pigment impurities for all metals.
[0251] Uniclean.RTM. 260 is a weak alkaline sodium hydroxide
electrolytic cleaner, having electric conductivity, for the use for
cathodic or for anodic degreasing.
[0252] Uniclean.RTM. 675 is an acidic activation agent for
universal use. This cleaner contains sodium hydrogensulfate and
sodium fluoride.
[0253] After having cleaned the substrate, the foil may be sent to
selenization and/or sulfidation.
[0254] In the following Examples, numerous specific details are set
forth in order to provide a thorough understanding of the present
invention. It will be understood, however, to one skilled in the
art, that the present invention may be practiced without some or
all of these specific details:
[0255] In one embodiment, the heating rate was 10 K/s until the
final temperature of 550.degree. C. as reached. This rate was a
compromise between reaching as soon as possible the necessary
sulfur partial pressure and the kinetics of the formation and
recrystallization of the (Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a
(1.5<x<2.5; 0.9<y<1.5; 0.5<z<1.1; 0.1<a
<4.2).
[0256] Formation of (Au,Ag,Cu).sub.xZn.sub.ySn.sub.zS.sub.a
(1.5<x<2.5; 0.9<y<1.5; 0.5<z<1.1; 0.1<a
<4.2) was linked with an expansion of the layer. For a
sufficient sulfurization and cure of the layers the final
temperature of 550.degree. C. was held for at least 60 min. A
homogeneous crystal structure was observed within 120 min reaction
time.
[0257] After the sulfurization step cooling started. The flow rate
of nitrogen was increased to 500 seem for a faster exchange of the
gas atmosphere. Until approximately 35O.degree. C. the graphite box
containing the samples was allowed to cool down without further
cooling devices. After that the cooling was supported by the fan
system of the furnace.
[0258] The recrystallized absorber layers were further processed to
a thin film solar cell using standard procedures known from the
copper indium gallium selenide system.
[0259] Any of the forgoing may be deposited by solution deposition
techniques, vacuum techniques, sputtering, evaporation, chemical
bath deposition (CBD), electroplating, sol-gel based coating, spray
coating, chemical vapor deposition (CVD), physical vapor deposition
(PVD), atomic layer deposition (ALD), and the like.
Size Reduction
[0260] The particles may be formed by using at least one of the
following methods: grinding, milling, electroexplosive wire (EEW)
processing, evaporation condensation (EC), pulsed plasma
processing, or combinations thereof. The particles may be formed
using at least one of the following methods: sonification,
agitation, electromagnetically mixing of a liquid metal or liquid
alloy. The particles may be formed using at least one of the
following methods: spray-pyrolysis, laser pyrolysis, or a bottom-up
technique like wet chemical approaches.
[0261] In one embodiment, the sample started out at feedstock with
spherical ternary CZT or ACZT particles of about 10.about.50
microns, then was filtered down to .about.10 microns, then milled
down to <4 micron in the 1st stage, then finally milled down to
1.about.2 micron in the 2nd stage.
[0262] Normally, particles to be made smaller involves using a
higher mill speed. In this case with CZT or ACZT, the faster the
mill got, the particles got larger and larger. As the mill speed
slowed down the mill, the particles got smaller. This is the
non-newtonian aspect of the CZT or ACZT particles. This could be
due to the specific density and formulation that were used.
[0263] The CZT is similar to cornstarch. Hit at high velocity, it
is hard. But push slowly it is soft. The higher the shear force,
the modulus gets higher. Thus, the CZT or ACZT precursor did not
behave as expected. Solution is to use a slower mill speed at the
beginning (not hard). Not 50% slower, but tens slower.
[0264] In one embodiment, grind pressure is 0.4 to 0.8 bar.
Optionally, grind pressure was reduced to 0.2 to 0.7 bar.
Optionally, many embodiments with about 0.2 to about 0.3 bar were
able to produce the desired size reduction while minimizing
agglomeration. Optionally, many embodiments with about 0.2 to about
0.4 bar were able to produce the desired size reduction while
minimizing agglomeration. The percent solid loading is between
about 4 to 6%. Optionally, they may be between 3% to about 10%. Run
times may be from 30 to 200 minutes in one embodiment. The samples
of particles may also be sonicated before, during, or after size
reduction to improve coating quality with the materials. The
material formed from the process is also in the form of flakes. The
flakes 450 in the figures may have a longest dimension in the range
of about 1 micron to 2 microns. Some embodiments may use smaller,
submicron flakes.
[0265] In one embodiment, Cu pct is in the range of about 38% to
40.5%; Sn pct is between 32% to about 38.2%; Zn pct is between 23%
to about 27%. The precursor material according to the present
invention may have material components in this range to produce the
desired final stoichiometric ratios in the final semiconductor
absorber.
[0266] While the invention has been described and illustrated with
reference to certain particular embodiments thereof, those skilled
in the art will appreciate that various adaptations, changes,
modifications, substitutions, deletions, or additions of procedures
and protocols may be made without departing from the spirit and
scope of the invention. For example, with any of the above
embodiments, traditional thermal annealing may also be used in
conjunction with laser annealing. For example, with any of the
above embodiments, microflakes may be replaced by and/or mixed with
nanoflakes wherein the lengths of the planar nanoflakes are about
500 nm to about 1 nm. As a nonlimiting example, the nanoflakes may
have lengths and/or largest lateral dimension of about 300 nm to
about 10 nm. In other embodiments, the nanoflakes may be of
thickness in the range of about 200 nm to about 20 nm. In another
embodiment, these nanoflakes may be of thickness in the range of
about 100 nm to about 10 nm. In one embodiment, these nanoflakes
may be of thickness in the range of about 200 nm to about 20 nm. As
mentioned, some embodiments of the invention may include both
microflakes and nanoflakes. Other may include flakes that are
exclusively in the size range of microflakes or the size range of
nanoflakes. With any of the above embodiments, the microflakes may
be replaced and/or combined with microrods which are substantially
linear, elongate members. Still further embodiments may combine
nanorods with microflakes in the precursor layer. The microrods may
have lengths between about 500 nm to about 1 nm. In another
embodiment, the nanorods may have lengths between about 500 nm and
20 nm. In yet another embodiment, the nanorods may have lengths
between about 300 nm and 30 nm. Any of the above embodiments may be
used on rigid substrate, flexible substrate, or a combinations of
the two such as but not limited to a flexible substrate that become
rigid during processing due to its material properties. In one
embodiment of the present invention, the particles may be plates
and/or discs and/or flakes and/or wires and/or rods of micro-sized
proportions. In another embodiment of the present invention, the
particles may be nanoplates and/or nanodiscs and/or nanoflakes
and/or nanowires and/or nanorods of nano-sized proportions. Again,
any of the foregoing may also be combined with spherical particles
in a suspension. Some embodiments may have all spherical particles,
all non-spherical particles, and/or mixtures of particles of
various shapes. It should be understood that the solid group
IIIA-based particles may be used in single or multiple combination
with particles of other shapes and/or composition. This may include
shapes such as but not limited to spherical, planar, flake, other
non-spherical, and/or single or multiple combinations of the
foregoing. As for materials, this may include alloys, elementals,
chalcogenides, inter-metallics, solid-solutions and/or single or
multiple combinations of the foregoing in any shape or form. Use of
solid particles with dispersions and/or emulsions of the foregoing
is also envisioned. The solid solutions are described in pending
U.S. patent application Ser. No. 10/474,259 and published as
US20040219730, fully incorporated herein by reference for all
purposes. The following applications are also fully incorporated
herein by reference: 11/395,438, 11/395,668, and 11/395,426 both
filed Mar. 30, 2006. Any of the embodiments described in those
applications may be adapted for use with the particles described
herein.
[0267] For any of the above embodiments, it should be understood
that in addition to the aforementioned, the temperature used during
annealing may also vary over different time periods of precursor
layer processing. As a nonlimiting example, the heating may occur
at a first temperature over an initial processing time period and
proceed to other temperatures for subsequent time periods of the
processing. Optionally, the method may include intentionally
creating one or more temperature dips so that, as a nonlimiting
example, the method comprises heating, cooling, heating, and
subsequent cooling. Some embodiments may use a two-step absorber
growth (non-reactive anneal for densification followed by reactive
anneal) without cool-down and ramp-up between densification and
selenization/sulfurization. Various heating methods, including not
heating the substrate, but only the precursor layer (laser) may be
used. Others heating techniques may use muffle heating, convection
heating, IR-heating. Some embodiments may use the same or different
techniques for heating the top surface and bottom surface of the
substrate. Basically, all heating mechanisms, being conduction,
convection, and radiation may be used. All temperature gradients
within the web (across the thickness), being uniformly heated from
bottom to top, and/or heating with a huge temperature gradient from
bottom (low T) to top (high T), e.g. with a laser, and covering all
web transport mechanisms through the furnace (including but not
limited to being free-span through the module, dragging over a
dense or partially open surface, or relying on a belt), orientation
of the furnace, horizontally, vertically, or anything in
between.
[0268] For any of the above embodiments, it is also possible to
have two or more elements of IB elements in the chalcogenide
particle and/or the resulting film. Although the description herein
uses an ink, it should be understood that in some embodiments, the
ink may have the consistency of a paste or slurry. It should be
understood that the deposition methods for use with depositing
precursor material(s) may include one or more of the following:
solution-deposition of particulates, like coating, printing, and
spraying, sol-gel, electro(less) deposition (HBP, CBD, e-Dep),
precipitations, (chemical) vapor deposition, sputtering,
evaporation, ion plating, extrusion, cladding, thermal spray, where
several of these methods can be plasma-enhanced) and
precursor/film-conversion methods, where the latter can be either
chemically, physically, and/or mechanically, and covers both
partial and complete changes of the precursor/film and/or surface
only.
[0269] Additionally, concentrations, amounts, and other numerical
data may be presented herein in a range format. It is to be
understood that such range format is used merely for convenience
and brevity and should be interpreted flexibly to include not only
the numerical values explicitly recited as the limits of the range,
but also to include all the individual numerical values or
sub-ranges encompassed within that range as if each numerical value
and sub-range is explicitly recited. For example, a size range of
about 1 nm to about 200 nm should be interpreted to include not
only the explicitly recited limits of about 1 nm and about 200 nm,
but also to include individual sizes such as 2 nm, 3 nm, 4 nm, and
sub-ranges such as 10 nm to 50 nm, 20 nm to 100 nm, etc. . . .
.
[0270] For example, still other embodiments of the present
invention may use a Cu--In precursor material wherein Cu--In
contributes less than about 50 percent of both Cu and In found in
the precursor material. The remaining amount is incorporated by
elemental form or by non IB-IIIA alloys. Thus, a Cu.sub.11In.sub.9
may be used with elemental Cu, In, and Ga to form a resulting
film.
[0271] In another embodiment, instead of elemental Cu, In, and Ga,
other materials such as Cu--Se, In--Se, and/or Ga--Se may be
substituted as source of the group IB or IIIA material. Optionally,
in another embodiment, the IB source may be any particle that
contains Cu without being alloyed with In and Ga (Cu, Cu--Se). The
IIIA source may be any particle that contains In without Cu
(In--Se, In--Ga--Se) or any particle that contains Ga without Cu
(Ga, Ga--Se, or In--Ga--Se). Other embodiments may have these
combinations of the IB material in a nitride or oxide form. Still
other embodiments may have these combinations of the IIIA material
in a nitride or oxide form.
[0272] The present invention may use any combination of elements
and/or selenides (binary, ternary, or multinary) may be used.
Optionally, some other embodiments may use oxides such as
In.sub.2O.sub.3 to add the desired amounts of materials. It should
be understood for any of the above embodiments that more than one
solid solution may be used, multi-phasic alloys, and/or more
general alloys may also be used. For any of the above embodiments,
the annealing process may also involve exposure of the compound
film to a gas such as H.sub.2, CO, N.sub.2, Ar, H.sub.2Se, Se
vapor, S vapor, or other group VIA containing vapor. There may be a
two stage process where there is an initial anneal in a non
group-VIA based atmosphere and then a second or more heating in
group VIA-based atmosphere. There may be a two stage process where
there is an initial anneal in a non group-VIA based atmosphere and
then a second heating in a non-group VIA based atmosphere, wherein
VIA material is placed directly on the stack for the second heating
and additional is the VIA-containing vapor is not used.
Alternatively, some may use a one stage process to create a final
film, or a multi-stage process where each heating step use a
different atmosphere.
[0273] It should also be understood that several intermediate solid
solutions may also be suitable for use according to the present
invention. As nonlimiting examples, a composition in the .delta.
phase for Cu--In (about 42.52 to about 44.3 wt % In) and/or a
composition between the .delta. phase for Cu--In and
Cu.sub.16In.sub.9 may be suitable inter-metallic materials for use
with the present invention to form a group IB-IIIA-VIA compound. It
should be understood that these inter-metallic materials may be
mixed with elemental or other materials such as Cu--Se, In--Se,
and/or Ga--Se to provide sources of the group IB or IIIA material
to reach the desired stoichiometric ratios in the final compound.
Other nonlimiting examples of inter-metallic material include
compositions of Cu--Ga containing the following phases:
.gamma..sub.1 (about 31.8 to about 39.8 wt % Ga), .gamma..sub.2
(about 36.0 to about 39.9 wt % Ga), .gamma..sub.3 (about 39.7 to
about -44.9 wt % Ga), the phase between .gamma..sub.2 and
.gamma..sub.3, the phase between the terminal solid solution and
.gamma..sub.1, and .theta. (about 66.7 to about 68.7 wt % Ga). For
Cu--Ga, a suitable composition is also found in the range in
between the terminal solid-solution of and the intermediate
solid-solution next to it. Advantageously, some of these
inter-metallic materials may be multi-phasic which are more likely
to lead to brittle materials that can be mechanically milled. Phase
diagrams for the following materials may be found in ASM Handbook,
Volume 3 Alloy Phase Diagrams (1992) by ASM International and fully
incorporated herein by reference for all purposes. Some specific
examples (fully incorporated herein by reference) may be found on
pages 2-168, 2-170, 2-176, 2-178, 2-208, 2-214, 2-257, and/or
2-259. It should also be understood that a particle may have
portions that are of a solid alloy and portions that are phase
separated into individual elements or other alloys that are
liquid.
[0274] It should be understood that any of the embodiments herein
may be adapted for use in a one step process, or a two step
process, or a multi-step process for forming a photovoltaic
absorber layer. One step processes do not require a second
follow-up process to convert the film into an absorber layer. A two
step process typically creates a film that uses a second process to
convert the film into an absorber layer. Additionally, some
embodiments may have anywhere from about 0 to about 5 wt % oxygen
in the shell.
[0275] It should be understood that the particles as described
herein may be used with solids, solid solutions, intermetallics,
nanoglobules, emulsions, nanoglobule, emulsion, or other types of
particles. It should also be understood that prior to deposition of
any material on the substrate, the metal foil may undergo
conditioning (cleaning, smoothening, and possible surface treatment
for subsequent steps), such as but not limited to corona cleaning,
wet chemical cleaning, plasma cleaning, ultrasmooth re-rolling,
electro-polishing, and/or CMP slurry polishing.
[0276] Furthermore, those of skill in the art will recognize that
any of the embodiments of the present invention can be applied to
almost any type of solar cell material and/or architecture. For
example, the absorber layer in the solar cell may be an absorber
layer comprised of copper-indium-gallium-selenium (for CIGS solar
cells), CdSe, CdTe, Cu(In,Ga)(S,Se).sub.2,
Cu(In,Ga,Al)(S,Se,Te).sub.2, and/or combinations of the above,
where the active materials are present in any of several forms
including but not limited to bulk materials, micro-particles,
nano-particles, or quantum dots. The ACIGS cells may be formed by
vacuum or non-vacuum processes. The processes may be one stage, two
stage, or multi-stage ACIGS processing techniques. Many of these
types of cells can be fabricated on flexible substrates.
[0277] In some embodiments, the method comprises depositing onto a
back contact of the substrate one or more films of elemental Ag,
Tl, or Te, or oxides, sulfides, selenides, or tellurides of any of
these. Subsequently one or more of Cu, In, Ga, Al, Se, or S is
deposited. Optionally, one or more of Ag, Tl, or Te may also be
deposited. This film may then be optionally be further processed if
necessary at a further elevated temperature in an inert or O-, S-,
Se-, or Te-containing atmosphere to form the chalcopyrite film.
[0278] In some embodiments, the method of making the film comprises
depositing one or more films of elemental Ag, Cu, In, Ga, Tl, Al,
Te, or alloys thereof, or oxides, sulfides, selenides, or
tellurides of any of these via sputter deposition or via reactive
sputter deposition in an oxygen-, sulfur-, selenium-, or
tellurium-containing atmosphere.
[0279] The method may instead comprise the deposition and
annealing, reaction, or sintering of a particulate chalcopyrite, or
precursor particles in a vacuum, inert, or S-, Se-, or
Te-containing atmosphere. Ag, Tl, or Te may be present in the
pre-processed particulate films either in elemental form or as
compounds.
[0280] Alternatively, Ag, Tl, or Te may be incorporated into the
I-III-VI.sub.2 absorber layer by simultaneous or sequential
co-evaporation with Cu, In, Ga, Al, Se, or S.
[0281] In some embodiments, Tl or its sulfides, selenides, or
tellurides are delivered to the substrate by thermal evaporation of
TIS, Tl.sub.2Se, Tl.sub.2Te, or other Tl sulfides, selenides, or
tellurides.
[0282] In another embodiment, an Ag film may be sputtered onto the
back contact, followed by the formation of the remainder of the
absorber layer by sequential or co-evaporation of Cu, Ga, In, Se,
and optionally additional Ag, to form a resultant
(AgCu)(InGa)Se.sub.2 absorber layer.
[0283] In other embodiments, Ag, Tl, or Te are incorporated into a
precursor film or films before annealing in a vacuum or inert
atmosphere, or reaction in an S-, Se-, or Te-containing atmosphere
to form the resultant I-III-VI.sub.2 absorber layer.
[0284] In yet other embodiments, Ag, Tl, or Te are incorporated
into the I-III-VI.sub.2 absorber layer by deposition onto a film
containing Cu, In, Ga, Al, Se, or S, and optionally Ag, Tl, or Te,
and then heating in an inert, vacuum, or S-, Se-, or Te-containing
atmosphere.
[0285] The method may also comprise sequentially co-evaporating Ag,
Cu, In, Ga, and Se onto a heated substrate to form the chalcopyrite
film. Or, it may involve depositing one or more layers of Ag, Cu,
In, Ga, and optionally Se, or alloys or oxides, sulfides, or
selenides thereof, and subsequently processing the film at a
further elevated temperature in an inert, O-, S-, or Se-containing
atmosphere to form the chalcopyrite film. Alternatively, the method
may include depositing a particulate film comprising Ag, Cu, Tl,
In, Ga, O, S, Se, or Te, or a combination thereof, or alloys or
oxides, sulfides, selenides, or tellurides thereof, and
subsequently processing the film at a further elevated temperature
to form the chalcopyrite film.
[0286] Suitable substrates upon which to dispose the absorber layer
films of this invention include any known in the art. Specific
examples include metal films, glasses (including soda-lime glass),
and self-supporting polymer films. The polymer films may for
example be polyimides, liquid crystal polymers, or rigid-rod
polymers. A typical film thickness may be about 50 .mu.m to about
125 .mu.m, although any thickness can be used.
[0287] Additionally, concentrations, amounts, and other numerical
data may be presented herein in a range format. It is to be
understood that such range format is used merely for convenience
and brevity and should be interpreted flexibly to include not only
the numerical values explicitly recited as the limits of the range,
but also to include all the individual numerical values or
sub-ranges encompassed within that range as if each numerical value
and sub-range is explicitly recited. For example, a thickness range
of about 1 nm to about 200 nm should be interpreted to include not
only the explicitly recited limits of about 1 nm and about 200 nm,
but also to include individual sizes such as but not limited to 2
nm, 3 nm, 4 nm, and sub-ranges such as 10 nm to 50 nm, 20 nm to 100
nm, etc. . . . .
[0288] The publications discussed or cited herein are provided
solely for their disclosure prior to the filing date of the present
application. Nothing herein is to be construed as an admission that
the present invention is not entitled to antedate such publication
by virtue of prior invention. Further, the dates of publication
provided may be different from the actual publication dates which
may need to be independently confirmed. All publications mentioned
herein are incorporated herein by reference to disclose and
describe the structures and/or methods in connection with which the
publications are cited. For example, US 20040219730 and US
2005/0183767 are fully incorporated herein by reference.
[0289] While the above is a complete description of the preferred
embodiment of the present invention, it is possible to use various
alternatives, modifications and equivalents. Therefore, the scope
of the present invention should be determined not with reference to
the above description but should, instead, be determined with
reference to the appended claims, along with their full scope of
equivalents. Any feature, whether preferred or not, may be combined
with any other feature, whether preferred or not. In the claims
that follow, the indefinite article "A", or "An" refers to a
quantity of one or more of the item following the article, except
where expressly stated otherwise. The appended claims are not to be
interpreted as including means-plus-function limitations, unless
such a limitation is explicitly recited in a given claim using the
phrase "means for."
* * * * *
References